The culprit is federal law 922(o): prohibition of civilians owning/building any machineguns made after 1986.
Worldwide, the leading edge of firearms development is "lots of tiny holes" a la the FN P90, HK MP7, and several other ground-up redesigns. The application is compact arms which can penetrate soft armor, solved by tiny projectiles in cartridges between the .22LR and 5.56x45, stored in 50-round magazines; the small size penetrates well, and low recoil + full auto + large magazines = lots of holes = sufficient damage via a compact package.
BUT ... we're not allowed to make any. Domestic manufacture of any arms is dwindling, with the only real advancements coming in high-priced long-range single-shot systems like the .408 CheyTac, and is driven/funded by civilian markets. There being no legal market for new full-auto products, and research requiring hard-to-get licenses, we're stuck tinkering with 50-year-old platforms and optimizing ultra-long-range rounds.
Overturn 922(o) - c'mon, Heller practically gave us the template - and you'll see new effective systems appear almost overnight.
What - and Belgian civilians get to purchase these? It has to be more than just that. The past 30 years have seen American firms in nearly every industry from machine tools to consumer electronics either collapse entirely or become shells of what they once were. Either way domestic innovation has stopped.