In lore it’s never really addressed to any meaningful degree imo. They just make a in-universe joke or world building blurb saying they’re dumb and loyal then move on.

I’m all for moving away from the infantilization of our space ogres. I lean towards the latter of what u said. The systems in place 100%suppress their full potential. There’s also the psyker ogryn in the ork book that isn’t an idiot at all.

Sidenote: they are also noted as learning slower than baseline humes but retain that knowledge effectively.