Generally speaking, "mag" wheels (ones made of aluminum alloy) are very heavy. While they are stronger, they are also stiffer, which is not entirely a good thing. Regardless, every design, every material, every component on a bicycle is a compromise of some sort. Most manufacturers do not use mag wheels because any benefits do not outweigh (no pun intended) the drawbacks. There are of course carbon mag wheels, but they are really expensive and primarily only used on track/velodrome bikes.
