I'm not sure that's true. Look at the printing press. Sure, Shakespeare is a classic, but most of the rest of what we read as "classics" probably centers around the mid-1800s (at least in English-speaking countries)
If anything this claim merely shows the effect of “time compression” in which distant events feel like they happened around the same time.
Also the 19th century is not by any means considered generally superior to any other period in English literary history. Eg Jane Austen and Virginia Woolf and James Joyce are looking sternly in your direction from either side of that timeline for example.