Dredd flopped because people don't care about Judge Dredd. It's really that simple. Most of the people in the U.S. only know the character from the awful Stallone movie, and they weren't going to return for another installment of something like that.
True, the marketing wasn't that great, but even if we'd been bombarded with TV spots and tie-ins, I don't think it would have had that much effect on the box office.
Robocop at least has a lot more name recognition than Dredd. However, the bad sequels and the fact that it's been a long time since the beloved original might keep it from appealing to the masses. Additionally, fans of the original might swear off the new one (some already have) simply because they don't want to see it remade.