It's just a guess but I believe that the rotors end up warping more when you have aluminum rims. The reason being is that the aluminum on the rim corrodes where it mounts up against the steel rotor. Since it doesn't corrode evenly, the lug nuts loosen up in different spots unevenly because of the gap that opened up between the rotor and rim. Maybe it you paint the mounting surface of the rim, it would help but then that might affect heat dissapation from rotor to rim. | 
