Any study of rig effectivness points out that on a per-area basis, jibs are more effective than mainsails. That leads to the conclusion that rigs would better if they were all jib - hence the mast aft rig. What these studies often fail to point out is the effectiveness of the jib comes from its interaction with the main. On its own, it doesn't perform as well. And theoretically the main suffers in the exchange, making it look worse. But the combination is more than either sail acting in isolation.
The finest article I've ever read on the interaction of multiple lifting surfaces is A.M.O. Smith's 1975 Wright Brother's Lecture, "High Lift Aerodynamics" (AIAA Journal of Aircraft, Vol. 12, No. 6, June 1975, pp. 501-530.). He goes into great detail on the five primary effects of slotted airfoils and how they contribute to high lift. Very illuminating with regard to the main/jib interaction.
Then there's the problem of the mast. It's almost impossible to fair an isolated mast because of the range in apparent wind angles. The apparent wind will meet the mast from 20 - 30 degrees either direction. No section shape for the mast will avoid massive separation under these conditions, causing a lot of drag. The drag of a circular cylinder can be the same as an airfoil ten times its thickness and a hundred times longer - with drag coefficients based greater than one. In other words, the drag of an isolated mast can be almost as much per square foot as the sail produces in lift per square foot! When you add up the frontal area of the mast plus any struts and rigging, you get a lot of windage.
The mainsail acts as a splitter plate behind the mast on a conventional rig, preventing the formation of a Karman vortex street in the wake of mast, and the favorable pressure gradient in the slot allows the flow to reattach to the mainsail, minimizing the separation behind the mast. Finally, the jib channels the airflow so that it hits the mast from a much narrower range of angles, making it feasible to reduce its drag by elongating its cross section. So the drag of the mast in a conventional rig is much less than the same mast standing by itself in a mast-aft rig.
Lastly there're the structural considerations. A mast aft rig, especially one that is inclined forward, makes it very difficult to maintain adequate tension in the forestay, leading to lots of sag. The forestay is longer, and the angle between forestay and mast is larger than the angle between backstay and mast. This means the loads on the backstay are far higher than the loads on the forestay, and the mast compression is much greater to achieve a forestay tension anywhere near that of the conventional rig. Making the mast larger, heavier, and needing additional reinforcement to stiffen the boat. You could mitigate some of these problems by mounting the mast more midships and raking it steeply aft to put the hounds at the same location. This would shift the mechanical advantage of the forestay and backstay.