Why the dramatic drop-off in the number of titles being published after the 1920s? Works published before 1923 are known to be in the public domain, giving publishers the right to republish them without asking anyone's permission. In contrast, works published since 1923 may still be under copyright protection, so publishers have to do legal research to find the copyright owner (if any) and negotiate a license. That's a hassle, so works published after 1923 are much less likely to be republished once they've gone out of print.
There is an important caveats to Heald's results. The pre-1923 books include many duplicates — different publishers offering competing copies of the classics. Heald estimates that the average pre-1923 book has four copies available on Amazon. But Heald finds that even correcting for that factor doesn't change the basic picture. There are still a lot more titles in print from the 1890s, 1900s and 1910s than from the 1930s, 1940s and 1950s. And that's especially surprising because the number of books being published per year was going up during the 20th century. So, there are a lot of books from the mid-20th century that would be more widely available if copyright law wasn't standing stand in the way.
Congress last extended copyright terms in 1998, adding 20 years to most terms. That means that works from 1923 are scheduled to fall into the public domain again on Jan. 1, 2019, unless Congress grants yet another extension before then.