I would guess there aren't as many because, in order to implement this, your language must already have an HTTP/3 library. My language of choice doesn't even support QUIC yet (so I'm writing the library for it, then for HTTP/3). I wouldn't be surprised if other languages are similar.
As of one month ago, Java still didn't have HTTP/3 support. Though it's apparently coming in March (with JDK 26).
But yeah the HTTP/3 integration definitely makes WebTransport harder to support. The QUIC connection needs to be shared between HTTP/3 and WebTransport.
