The reason is that flat bottom wings fight you so much of the time. They are very speed sensitive, and really are holdovers from the days when RC airplanes flew themselves with occasional interferrence from the pilot. If you have a good instructor, and buddy box setup, a much better airplane will have a semi-symmetrical or symmetrical wing and a low wing loading. Something along the lines of the H9 Arrow, or any Ugly Stik, or a Goldberg Sr Falcon flys much better than the typical trainer.

If you must have the flat bottom wing, the Sig LT-40 is the best of the lot. I have flown them all with an endless stream of beginners. In total fairness, the worst of the lot is any airplane with dura in the name.