The Atari 2600 homebrewers release the roms for their games all the time when they go out of print.
And there are at least
a thousand times more NES collectors than 2600 collectors, and I bet you that 90% of them can be duped into buying a bootleg cart release of a limited edition game because they don't know better. 2600 collectors, on the other hand, do know better since they're much more niche and a tighter community.
Not to mention it's much cheaper to make NES bootlegs than 2600 bootlegs. Anyone can take a game and slap it on a new, mass-produced NES board. Or if they're hacks, a quick ROM swap and boom, new game for sale. I hate repro hacks and stuff but even I think that these limited edition carts are fine because they don't get pirated to hell and back.
If these guys really wanted to have their games played by everyone then they'd release the roms, true, but they don't, very likely for the reasons I mentioned - and that's their choice.