It’s largely due to a move from single-branch designs to weapons “platforms” that are used by multiple branches of the armed forces so they now go through far more bureaucratic design processes. They had to do it because of the funding cuts after the end of the Cold War (although some systems like ATACMS predate that, both it and HIMARS just reflect a more bureaucratic army). HMMWV and MRAP though are generic terms, not really restricted to the US armed forces.

Designs that are ordered by a single branch like the USAF often still have the emotive names like the Predator and Reaper drones.