It is simple. Most players using two handed forehand were very weak when they were children. In two handed forehand mostly one hand is active(right) and in two handed backhand two hands are active. You see that Hradecka who is strong has powerful two handed forehand. We could only imagine how fast Nadal or Federer would hit the ball if they used two handed forehand.
To change technique of playing tennis in the whole world one Seles is not enough. What is equally important. I wouldnt advise to learn her forehand. There are much better two handed forehands (Peng, Hradecka, Bartoli).