I think one reason is that some people, including me, find Slack's behavior of not moving threads with activity up extremely irritating. It means that thread replies are very easy to not notice and it makes it confusing to understand who knew what when.

I realize this is a matter of opinion but I think that's kind of the problem, both platforms have chosen one way and forced it on users. I'm not sure that there's any same way to make this configurable either, but I don't think one option is clearly Superior to the other.

There's also plenty of precedent either way, even just looking at old forum software which tended to sort topics by activity (the Teams approach). That seems to mean that neither way is going to be obvious to all users.

I guess the conclusion is that I'm not sure there's an approach to this problem that won't piss someone off, but by that token it's not a clear win for Slack.

Also for what it's worth I think Slack's behavior is the less common option across similar products, but I've not done any kind of exhaustive survey, I just use 4-5 of these things and Slack is the one that always throws me off by behaving that way.