Probably because Stock works best for Smash, so many players automatically think it works best for PSASBR too.
Even though I agree Kill Limit is a better fit for this game than Stock, I still think Timed matches destroy them both. This battle system seems very balanced towards timed matches, and I'd probably stop ignoring Kill Limit and Stock if SSM actually does add a timer to those modes.
Kill limit is fun too. I think it makes the most sense for this game. Since killing with supers is the only way to score points.
Stock is fun as well, but much less so. It becomes about survival more than killing. It works better for Smash in my opinion.