US military doesn't like bullpups for some reason.
It has probably been swayed by the early experiences with the bullpup design in other armed forces.
The FAMAS for example is truly horrible.
I had the dubious honour of using the L85-A1 and frankly it was a fairly shocking piece of crap.
It was rattly and unnecessarily heavy, the cheek pad melted in contact with Army issue insect repellant and repellant containing cam-cream. It was possible to load two rounds, one behind the other, because the extractor in the bolt head would hold onto the first round, when fired the front round would head off down range, the rearmost round would more often than not come out of the top of the receiver, with potentially fatal results.
Field stripping took a good amount of time, parts like the firing pin retaining pin were tiny and easily lost. Failing to place the rear trigger mechanism housing locking pin exactly in the correct place would result in the main spring and guide shooting out into a bush or up your nose, and the gas plug could get permanently jammed if re-inserted at the wrong angle.
The magazine catch was positioned in such a way that with the correct sling position, it would bang against you chest and dump the mag.
The H+K revised A2 is an exemplary weapon however, which is accurate over iron sights out past 600m, the magazine change is intuitive and quick, the weapon itself is now pretty soldier proof.
I like the bullpup design, the only real issue with the bullpup that I can find is that I wouldn't fancy bayonet fencing with a weapon that short