I agree, James. Mice and rats breed the fastest and easiest, with large litters, so the cost is lower. I also think that many people would have a harder time feeding a hamster or gerbil, since they are probably seen as more of a "pet". Yes, I know Lori, mice are pets too.
Other than that, I'm sure they could be used. I know one of my buddies has a snake that will only eat Russian Dwarf hamsters.
It also helps that they have a tail- much easier to grab without getting bit.