Is there a reason why Internet Explorer chose not to support SMIL Animations? Will future versions of Internet Explorer support SMIL animations?
According to both the caniuse.com page you linked to and the official status page for IE and Edge, Microsoft is not planning to add SMIL to IE or Edge anytime soon. Actually, even if it came to Edge it would probably still not come to IE since IE's primary purpose in Windows 10 is legacy compatibility. As for why, I don't know for sure. This (old) answer suggests that Microsoft had some objection to the spec, but since that was 5 years ago that might not be the case anymore.
Microsoft believes that there are spec issues to work out before they add SMIL
Additionally, this IE blog post from back when they implemented SVG in IE 9 explains their reasoning at the time for not including SMIL support:
[...] support for SMIL animation of SVG in the web development community is far from strong. The leader of the SVG standardization effort wrote that not supporting SMIL in its current state is probably best “since the SVG WG intends to coordinate with the CSS WG to make some changes to animation and to extend filters.” There’s already work started to reconcile CSS3 animations and SVG.
That makes it sound like they were expecting the spec to change and they wanted it to settle down before implementing it. That way, they wouldn't get stuck shipping an obsolete implementation. And perhaps by now the spec is stable enough to implement, but not considered worth the effort anymore.