Why did "insofar" become a word, not "insofaras"?

Solution 1:

Merriam-Webster cites that insofar can also be found with that apart from as. In both cases insofar is listed as a conjunction. Maybe that could answer your question: as isn't the only word following insofar.

...cooperated fully insofar that many of their projects were jointly conducted.

Solution 2:

I think the question would be, how many words can we put into one word and not go crazy trying to decipher what it means. It seems like the max is three. Can you think of a word with four? Nothing comes to my mind right now.

Solution 3:

In-so-far or insofar is a problematic word. Here is what Henry Watson Fowler says in his "Dictionary of Modern English Usage" (1926)

The safest way of dealing with in so far is to keep clear of it. The dangers range from mere feebleness or wordiness, through pleonasm or confusion of forms, & inaccuracy of meaning, to false grammar.

(For examples follow the link provided)

So, examining the examples it is obvious that even the word insofar has a problem with both usage and the meaning (also evident in dictionary definitions).

I believe that adding a suffix as would even worsen the situation and that it would be really hard to see what is meant by the term insofaras and that this is the principal reason for not having that as a word.

(note: Also notice one more thing very nicely shown on your 3^rd ngram - the gap between "in so far" and "in so far as" had steadily diminished ever since insofar had been established as a term)