Since http1 webservers are usually stuck there for a (more or less) good reason, they won't be converting to http3 anytime soon, while already more modern webservers are easier to upgrade from http2 to 3.
HTTP1: the OG, meant for a much smaller individual scale. So great even today for smaller websites. Where smaller right now, with modern hardware, can actually still mean hundreds of requests per second and millions per day...
HTTP2/HTTP3: BigCorp optimization primarily for problems they're facing and not that many others are.
So viewed through this lens it's clear what's happening.
BigCorps are just moving from HTTP2 to HTTP3 and mom and pops don't give a crap, HTTP1 is good enough.