The reasons are simple, most people weigh less than that so it's only limiting in that scenario. Another reason is that the 150/152 is cheap to run, simple enough to fly but with enough quirks to be valuable for training purposes. It's rugged enough to take the daily knocks it receives in flying schools. Some more recent 'sexy' trainers have a habit of breaking things.........and there's still a lot of them around.
The MAUW is limiting with big men on board. Most reputable flying schools point the bigger student towards the 172 as a trainer in that case. I've seen a 150 try to take off with two big guys on board. It just won't. I don't know why they even tried.