I don't follow evolution of those protocols, but i am not sure how disallowing custom certificates has anything with "evolvability" of protocol ...

One of the reasons for developing HTTP 2 and 3 was because it was so difficult to make changes to HTTP 1.1 because of middleware that relied heavily on implementation details, so it was hard to tweak things without inadvertently breaking people. They're trying to avoid a similar situation with newer versions.