Originally Posted by The_Dunedan

Because it tends to unreliability, is less accurate than the AR by quite a ways (and MilSpec for the M4 and M16 is 4MOA; most of the inherant accuracy advantage lies in the AR's superb sights and trigger), doesn't accept optics well because of the open receiver, gets filled with crap for the same reason, it's trigger sucks compared to the AR/M16, lacks aftermarket support, lacks suitable numbers of standard-capacity magazines, and will spew hot gases and such directly back into the users' face in the event of a slamfire or pierced primer, again due to the open receiver. I'm not an AR fan, but it's a vastly superior weapon to the Mini.