A couple of possible reasons...

Harder to keep the ropes under control on a beach cat with nets rather than tramps. They keep slipping through the holes, dragging in the water and being annoying to use. The bigger boats can keep most of their ropes on the hulls. I have centreline lacing at the moment on my beach cat and it will be changed to a proper tramp soon, mainsheet keeps getting sucked through and causing dramas.

Windage in relation to capsizing would be significant on the big boats, particularly in the predominantly offshore racing they do, if they had tramps rather than nets. Not so important for off-the-beach cats racing inshore.