CRAMs are both several tons in weight, require a lot of power and special mounting, and are found mounted on a warship or in a military base; you're not really going to worry about some random insurgent carrying it off. Besides, they eat 20x102mm ammunition, which isn't exactly common stuff.

That is a far cry from the consequences of leaving a M240 around, which is both man portable and consumes the incredibly common 7.62x51mm NATO cartridge.