Depends how old were talking but, I suspect the main reason is to do with the education of lower classes (that is, of anyone who wasn't both upper class and male). Throughout most of history only the elite were educated in terms of literature, and so most writing would have come from, and been intended for, this relatively small group. This would mean that writing was only for a select few, and thus probably tried to cater to this elitist attitude by writing in a more complex style. It is interesting to remember that Charles Dickens, whose works were very simplistic for the time, was widely criticised by contemporary writers for not using enough fancy language.

Nowadays, since most people can read and write, the profitability of literature relies on how many people can appreciate it and, since most people - though educated - don't necessarily know the meaning of big 19 syllable words, literature written in simpler English tends to sell better; for the modern person its just more enjoyable. That is why books like the Harry Potter Series, or the 50 Shades series, sell better nowadays than things like Shakespeare.