According to the caniuse.com site you are associated with and the official status page for IE and Edge , Microsoft has no plans to add SMIL to IE or Edge anytime soon. In fact, even if he came to Edge, he probably still wouldn’t end up in IE, since IE’s main goal in Windows 10 is legacy compatibility. And why I do not know for sure. This (old) answer suggests that Microsoft had some objection to the specification, but since it was 5 years ago, this may not be the case.
Microsoft believes that there are special problems for development before adding SMIL
Also, this IE blog post when they implemented SVG in IE 9 explains their reasoning when it didn't include SMIL support:
[...] SMIL SVG animation support in the web development community is far from strong. The head of the SVG standardization effort wrote that without maintaining SMIL in its current state, it is probably best, "since the WGS SVG intends to coordinate with the CSS working group to make some changes to the animation and extend the filters." Work has already begun on aligning CSS3 and SVG animations.
It sounds like they were expecting a specification change, and they wanted it to calm down before it was implemented. Thus, they will not stop supplying an obsolete implementation. And, perhaps, by now the specification is stable enough for implementation, but is no longer considered spent on it.