What skews our evidence about 19th century English writing?

Your original premise is a little flawed – at least in the UK. The standard of written English of the Victorian lower classes was, prior to free schooling, particularly low. A cursory examination of the small amount of written English between members of the working class will bear this out.

The 19th century English that has come down to us, via our own education, is mainly the writings of the upper and (small) middle classes. We thus are mainly dealing with the idiolect of the upper and middle classes of the Victorian British.

The next thing was that writing was done for two motives: 1 Money and 2 Personal contact.

Money

People wrote business English and legal English. These people were wealthy.

People wrote books that only the wealthy, and therefore educated, could afford.

Politicians needed to be wealthy and well-educated as, by and large they represented the interests of the wealthy and well-educated.

Newspapers (in the early part of the century) were owned and bought by the wealthy and well-educated. The “Penny-dreadfuls” and popular songs of the working classes were not written in what would, today, be called “formal English” at all.

Advertising in newspapers was of little use unless it addressed the wealthy and well-educated in their own language. You will note that [pseudo-]scientific and [pseudo-]medical terms abound, and yet only the wealthy and well-educated knew anything about science and medicine.

Advertising was paid for by traders. The position of traders in society was ambivalent.

There was a group of traders who were seen as desirable and trustworthy purveyors of good quality goods and services to the sons and daughters of gentlefolk. These were mainly businesses owned and run by the upper and middle classes for the upper and middle classes.

Another group of traders were seen simple merchants who were little better than the poor to whose class they belonged. These were viewed with some suspicion by the upper and middle classes but were the foundation of an economically, but rarely socially, mobile class.

The final group were rarely encountered by the wealthy and well-educated. They were very small businesses serving the local working classes. Many would have been all but illiterate and would not have advertised in newspapers.

In order to benefit from the patronage of the upper and middle classes – the wealthy and well-educated - traders had to give the impression that they were trustworthy. To do this, they used English in what they saw as an educated way and combined overtones of subservience and deference that their potential customers - who were wealthy and well-educated - expected.

As the 1800s progressed, so did the Industrial Revolution, the poor eventually saw an increase in living standards, and thus money that they could spend on non-essentials. In effect, the lower classes became customers for larger businesses. Added to this there was free schooling, free libraries, popular newspapers, etc., all aimed at their class. And the infrastructure improved and thus greater travel became practical for them.

This all created the social conditions for a relaxation of the stilted English idiolect of the upper and middle classes, and the tendency/need to address the lower classes in their own language.

Personal Contact

The upper (and to a lesser extent the middle classes) were familiar with travel and also, mainly the men, had been educated at boarding schools and had childhood friends scattered throughout Britain and her colonies. They communicated in the language that they had been taught by teachers 20 to 50 years their senior who, themselves had been classically educated. It was small wonder that their idiolect comprised a wide vocabulary and the expansive translated terms of Latin and Greek literature.

The lower classes had little need to read and write and when they did, it was a few words to people similar to themselves. They knew nothing of foreign languages and literature. Their social circle was, by and large, restricted to the area in which they were born.

In the final quarter of the 1800s, free schools contributed a lot to the change in English. The schools were mainly staffed by ex-pupils (professional teachers were relatively rare) and so the idiolect of the now basically-educated-lower classes became popularised. The working classes travelled – and that required their skills in writing to send post cards and then there were letters between friends separated by one of the wars or distant employment.

Thus, from the start of the 20th century, two wars and a massive increase in the infrastructure and communications, changed society and, to an extent the class structure and the plainer language of the common man has come to dominate.

EDIT

@Elaine Nai

It does not explain why EVEN the highly educated, among themselves, speak nowadays much less formally than their counterparts in the 19th century.

I may not have been clear enough: 18th century English was very much a society divided, in all respects, by wealth and class.

I wrote

The 19th century English that has come down to us, via our own education, is mainly the writings of the upper and (small) middle classes. We thus are mainly dealing with the idiolect of the upper and middle classes of the Victorian British.

to which I added

[The upper classes] communicated in the language that they had been taught by teachers 20 to 50 years their senior who, themselves had been classically educated. It was small wonder that their idiolect comprised a wide vocabulary and the expansive translated terms of Latin and Greek literature.

You can add to this that the English taught to the upper classes at their fee-paying schools was very prescriptive and broadly founded on a belief that Latin grammar was the basis for explaining English - the lower classes had no such education. In effect, this was continuing a tradition of maintaining the idiolect of the upper classes.

In effect, in the 19th century, England spoke two "Englishes". That of the educated classes and an amazing variety of dialects, including some that would have been all but incomprehensible to another person from 100 miles away. Since World War Two, these dialects have died out or have been standardised to "Standard English" in which the vocabulary, grammar and syntax have been slowly moulded into one language, chiefly differentiated only by accent.

Also since World War Two, society has become somewhat more egalitarian and the ease with which everyone can communicate has been remarkable. From this, we see that the majority of communication is done by and for the benefit of the "common man." This has been and is done in his idiolect so as better to communicate with him. The motive has been money. People want to sell, to the "common man". Thus, newspapers, books, TV and radio programs and adverts, songs, etc., are written and spoken in that language and a whole slew of popular goods are sold that way.

So why would the upper classes/nobility use the argot of the lower classes? I suggest that there were two main factors: 1. The post-war, general feeling of the lower classes that they were as good as upper classes and no longer needed to be subservient to them or treat them as authorities. 2. The constant exposure of the upper classes to the ideas, lives and language of the lower classes.

The result has been that the upper classes moderated their social ideas and moderated their idiolect. Their schooling was more informed and broader.

As a generalisation, the speech of the upper and upper-middle classes (broadly, those who are educated at fee-paying schools) currently has much the same vocabulary and structure as that of the common man - it is distinguished by the [often] RP accent in which it is spoken, and the lesser use of common colloquialisms.

You may wish to compare the transition from

(i)Old English prior to the Norman invasion of 1066 when Norman French became the language of the ruling classes.

(ii) The parallel use of both languages – the upper class spoke French; the common man spoke Old English.

(iv) The decline of French in the upper classes.

(v) The amalgamation of both languages within Middle English.

(iii) The establishment of Modern English.