If you read the follow-ups, it's not just that no-one used TLS 1.2 because it wasn't available, enabling support for it actually broke stuff for end users[1] - and the browser developers knew this was almost certain to be the case even without looking, because it almost always is.
Also, as the linked email points out, we shouldn't have needed TLS 1.2 in the first place in order to be secure. It was already known at the time TLS 1.0 was designed that they were doing things in ways likely to be insecure for no good reason, but they did it anyway.
[1] https://www.ietf.org/mail-archive/web/tls/current/msg10614.h...
The follow-up comments paint a much fuller picture of why things are delayed, where the failures are, and what's going on.