I think that's not the case, at all. But if you select SOME kinds of music, it might seem that way.

Vocal music with lyrics was not rare at all before the 20th century. The most ancient forms of musical notation we have deal with vocal music, vocal music was preeminent in the so called middle ages and the renaissance. It was a very big deal in the 17th, 18th and 19th centuries. A very big percentage of the famous composers in the canon of classical music composed some kind of vocal music with lyrics.

If anything, there's less vocal music in classical music composed since the 1900s...

If you consider pre-20th century to be only instrumental classical music (symphonies, concertos), and post 20th century to be only non-classical music of the kind most people listen to these days (songs)... But that's not how things were.