Other then cost and the fact that it's easier to maintain?

Why do they insist on a single stage, every  weapon I own including WW2 rifles are all two stage.

What gives?

I believe a single sage gives a highe rate of fire than a 2 stage trigger

ever try double tapping with a 2 staged trigger?

yes, it work fine.

The military single stage is probably a result of the action of a select fire system.

If they had had their way, the AR/M16 series probably would have had a two stage pull, but remember the M16 wasn't designed by Army Ordanance. In fact, they didn't want it.  The M16 was first put into US service by the Air Force, on the direct orders of General Curtis Lemay, who did an end run around the procurment process in ordering the weapon for AF base security units.  Shortly thereafter, Defence Secretary Robert MacNemara ordered the rest of the armed forces to adopt th M16, over the screams and howls of Army Ordnance.  I suppose they could have designed a two stage trigger at that time, but it probably would have meant a major redesign of the full auto mechanism.