I've got to imagine that the cost versus payoff for a Colt-clone H&R LMG is not worth it. I love my retro AR builds and they include a 601, 604, 607, 610, 629 among others... (and I've spent thousands of $$ - quite literally - over the last couple months on H&R stuff) but frankly, a Colt LMG clone hasn't ever interested me.
Not knowing a ton about the 750, it seems like there are a lot of parts that would be unique to that specific gun (rather than other AR's) and would only sell for building that specific gun, which translates to them being pretty expensive to tool up and produce given the likely smaller market share and interest. Probably the same reason H&R doesn't offer a 607 stock or 607 specific parts outside of the receiver (which only requires different labeling, so not expensive).