Edge doesn't have an infinite budget, and they say they've tried to prioritize based on how common the standards are found used in the wild and how active the requests on UserVoice are. Doesn't look like there is even currently a tracked UserVoice for it:
It is on UserVoice [1], and it has over two thousand votes there. They've marked that as "sorry, no plans," and they've marked previous issues as "by design" [2]. They use Chrome announcing plans to deprecate SVG animations as their primary justification, but Chrome backtracked on those plans after realizing that there were important use cases that mattered to the community [3].
> In the 15 months since we announced our intention to deprecate and eventually remove SMIL, we’ve heard a variety of opinions from members of the community. We value all of your feedback, and it's clear that there are use cases serviced by SMIL that just don’t have high-fidelity replacements yet. As a result, we’ve decided to suspend our intent to deprecate and take smaller steps toward other options.
I understand that they don't have an infinite budget, but it really seems like they made their mind up here years ago and haven't been interested in listening in the community since then. It's also a bit circular to "prioritize based on how common the standards are found used in the wild" when they're single-handedly responsible for preventing developers from using SVG animations "in the wild" in the first place.
It is fascinating the interplay between the Chrome team, and WHATWG on SMIL. It's one of those W3C standards from the W3C's "XML all the things" years that you can tell browser teams aren't entirely thrilled with. Quick searches seem to indicate that WHATWG's overall take seems to be that all SVG animation support would be better in CSS Animation and that they'd also prefer to punt SMIL.
> It's also a bit circular to "prioritize based on how common the standards are found used in the wild" when they're single-handedly responsible for preventing developers from using SVG animations "in the wild" in the first place.
It certainly is a chicken-and-egg bootstrapping problem. Though the increasing prevalence of "Works Best in Chrome"/"Works Only in Chrome" dominance may suggest that Edge is still in a position to follow the trends than attempt to lead them on ancient W3C standards like SMIL.
And here I thought the Edge team left behind the mistakes of IE.