I think because sculptures and graphic work were able to be duplicated in most cases. May be thats why? May be painting was the accumulation of all knowledge drawing, composition, design, painting skills.. that is that was the case traditionally. Nowadays, who knows..
well, as far as a collector's side z concerned, i believe paintings are easier to store and display and they give full art and design satisfaction besides. Thats y. Otherwise i believe these two art form satisfy an art lover in their own different ways. We cant really say paintings are better than 3D art forms in all cases