It is probably easier to get molds for the handles made (last a loooong time) and the Res-c material is cheaper than machining the G-10 and Micarta. One person can probably make XXX handles on knives out of Res-c (time wise) to only XX for G-10 and Micarta. The Res-C is probably MUCH cheaper in bulk that the other two.

All this is of course speculation as I have NO idea how they do their handles/handle material. Heck It may even be bought with a certain profile for the knife model.