OOU bullpups on models are just more difficult to pull off because the posing and shape of the arm has to accommodate the magazine pressing into it. It makes weapon swap conversions harder.
It's also makes the model a little harder to visually read since the big chunky magazine that helps identify the weapon is hidden from most angles.
40k has had a few bullpups like Elysian lasguns, but that is a very niche model range. I swear I've seen a bullpup bolter on a character somewhere, but overall it is usually better to go with another design for tabletop reasons.