There will be factors such as purchase price, maintenance costs which may well be cheaper than other training aircraft. The high-wing design of the C150 and C152s is a help to trainees as well as it gives a good frame of reference to the ground, and helps with navigation and PFLs as you get a much greater view that with low wing aircraft.
I qualified for my PPL 18 years ago on C150s and 152s and realy enjoyed flying them, although am going to have to swap to a low wing now for my PPL retraining.