There are so many comments already and someone probably covered it better than me.

But passing the guard and maintaining position without the gi is usually harder than in the gi, and when people don't have to scramble to escape or defend things like getting their guard passed or their back taken (because of no need to worry about getting scored on), then there are less submission options for the top person.

Same with sweeps. If you sweep someone, it's usually to pass them so that you can sub them or take their back and sub them. But without points, why bother?

So instead, in these no point matches, you are seeing people do heel hooks. Since passing the guard takes a lot of effort and there may not be as many good openings for submissions during your pass, or during your back take, then it's a smart idea to just go for the legs.

It used to be that people would just dive for the legs. But the best leg lockers use very specific "leg entanglements" that control the other person. So going for the legs if done right can still give you the control of an upper body pin or back control, and give you just a good a chance of finishing the match.