Dredd failed because it had no advertising, because very few people know of or care about the character, because it had no well known actors attached to it, was released only in 3D in cinemas, and because it was pretty much a stinky plotless turd of a film. I don't think it failed because it was an 18.
The Raid did Dredd far better, not to mention did it first, and TBH, I'm not sure how I managed to get to the end of that film either. Utterly generic load of choreographed Asian chop socky.
Dredd had shit loads of advertising, there were billboards and it constantly advertised on TV. In the UK at least, lack of advertising internationally may have been a factor
Carl Urban was in Star Trek and Lord of The Rings, he's hardly a nobody.
It has a 78% on Rottern Tomatoes and 59/100 on Metacritic (one more than The Hobbit), reviews/the film aren't to blame.
Released only in 3D was probably a factor in the UK, don't know about the situation internationally.