[munificent]

I'm just watching this as an observer, but I suspect a lot of it is:

People who want to write real-world Scheme code have a strong incentive for a single definition of the language so that their code will reliably run on multiple implementations. They want one language and are probably willing to deal with some breakage in places where implementations are alreaday incompatible in order to get there.

People who maintain Scheme implementations want backwards compatibility so that they aren't forced to rewrite a bunch of already working implementation code or try to support multiple different languages at the same time. They don't want to make breaking changes to their implementations and break all of their users in order to converge towards some other implementations' behavior they may not care about.

Teachers want a language that is maximally elegant and minimal. They are perfectly happy to make tweaks to the language to make it incrementally cleaner even if it breaks tons of existing code because most of their code is short-lived anyway.

Reconciling these very different incentives does not sound fun.

[fuzztester]

Like herding wild, feral and domestic cats - all at the same time.

https://en.m.wikipedia.org/wiki/Cat_Herders

[johncowan]

Very well said. A fourth group is people maintaining existing user code (including libraries), who also want backward compatibility.