Part of the reason I believe, is that not many companies supply such things to movies. If one company does most of its work with a couple big movie houses, then they will generally be using the same guns from movie to movie. Plus when having to equip large numbers, such as many soldiers, or many cops, it is probably easier to go with a select few variants, with only speciallized ones for low volume roles, such as the gold looking ones in that travolta, cage movie, or the BG's in a MI movie. thats my guess anyway