Well, that's because superheroes are generally marketted towards children when they aren't in their comic book source material, and not only is Judge Dredd apt to kill the enemies he's dealing with with startling efficiency, but grappling with the morality of a dystopian future where a select few are taken from birth in order to train for one day being given complete moral and legal authority over all criminals to do whatever they deem right isn't something I think most kiddies are prepared for.
That isn't to say they couldn't pull a Robocop and make a kid's toy line for some obviously R-rated source material, but Dredd doesn't have that many main villains, and when he does, they either look unremarkable or they're grossly mutilated to some degree, (Which wouldn't have been a problem in the 80s and 90s. I mean, look at all the graphic shit that went down in TMNT and that one show about dinosaurs from hell! Too bad we live in an age populated by prudish parents who were traumatized by the 80s and 90s...) so the only options you'd really have are Judge Dredd and not much else.