Another factor may be who is reading. The world was a very different place back then.
Reading is much more accessible these days. Publishers offer more choice. People have more disposable income. Higher levels of education are attained. Literacy is expected of far more people. This created a huge shift in who reads, which would create a shift in what they read.
(As an aside: it is interesting to look at different measurements of literacy. Frequently cited statistics for western nations are above 95%. Those numbers drop below 80% if you expect people to do anything more than read and fill in forms. Assuming that I'm interpreting the report correctly, about half of the population doesn't have the skills necessary to read a novel.)