Knocking an opponent prone works a lot better with teamwork. All it takes is one orc to knock someone down and then their buddies spend their turns walloping on them. If you add an opportunity attack in, it becomes just not worth the risk of getting multiple OAs on you. I remember playing a Pathfinder 1e game where the whole of the party spent the last battle prone on the floor, rather than risk the opportunity attack. It was not fun at all, and I know it made me feel decidedly unheroic.
Switching things around, for a DM it would feel pretty disappointing to have your BBEG get surrounded, knocked prone, then have to trigger a bunch of OAs to get back up.