Why has the word "thrice" fallen out of common usage?

Solution 1:

Google Ngram shows the usage more or less steadily falling from 1810, when it was almost 9 times more common than now. From the shape of the graph, one gets the impression that it was more common still earlier to that.

It is a lovely word, easy on the ear.

"Before the cock crow twice, thou shalt deny me thrice," is not only in the King James Version, but in other translations as well.

Solution 2:

English taught and spoken as a second or third language in most former British colonies tends to be more florid and formal (old-fashioned if you like) than the ever-changing versions spoken in countries where English is the Mother Tongue.

My father-in-law is a very well educated Kenyan and I relish reading his letters. Some of the vocabulary is near Dickensian and the wit heavily influenced by Wodehouse.