Apparently because of compatibility issues. Found this via the Wikipedia page about this feature.
this post was submitted on 24 Oct 2023
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Protocol ossification is a huge problem. That's one of the reasons why the IETF went with the UDP based QUIC for HTTP/3.
Interesting read, thanks for sharing!
This article is so vague that if you aren't actually familiar with the problem they are describing, you will read the entire thing and leave yourself no more knowledgeable then when you began. But yea, I agree, the phenomenon they are describing is a huge problem.