Everyone has to be onboard before the switch can be made, and not everyone is happy about the somewhat messy solution of running dual-stack RSA+Ed25519 setups in the interim - it's a bit different than e.g. supporting multiple methods for key exchange or multiple ciphers for symmetric crypto. It's just one of those things that take time to happen.
If the big players (gmail, outlook) jump onto it the rest will be forced to follow. Outlook would probably jump in with a checkbox, and perhaps gmail will make at an option for the paid tier while everyone on the free tier gets no choice - but that is still enough. SmallCompany.com cannot do it alone, probably not even a fortune100.com (if any of them even care - their sales staff probably will overrule the few who do), but there are enough players if they all agree on something.
Getting the big players to agree and execute though is a lot like herding cats. I'm sure some in the big players are trying.
