Harry Potter - Popular, but never to be a Classic

[Quoth]

In the Victorian era it was cheaper to see a play about the Monster than buy Mary Shelley's book.
Cheaper books came in in late 1930s.
Libraries were much more important in the past. Stupidly the poorest areas here have small or no library and larger ones in well off areas.

Another aspect is literacy. It varies with time, place and parents. Children of parents that don't read are less likely to read well or have books in the house.