There are much more active subreddits that side with SRS if you want to go there, like circlebroke. There are also much more extreme subreddits than the_donald, like TheNewRight. Nobody is talking about banning those subreddits. This isn't a partisan argument. The_Donald is (purposely) interrupting how reddit works. I'm not an American and I have no dog in this fight, but the_donald is ruining reddit for a lot of us.
This has been happening for years, and the admins are complicit in it. As far as 'ruining reddit' goes, SRS/SRD are among the worst offenders for sure.
The admins aren't even on their side (their political leanings are assuredly different); SRS were just never as bad as people claim, and so the admins never had a reason to ban it.