All due to them being movie-based. For example, take a look at the Batman series vs the Avengers:
Batman vs Scarecrow/Joker/Two-Face/Catwoman/Bane
compared to
Ironman/Thor/Cap/Hawkeye/Widow/Hulk/Fury vs Loki
It's not that easy. It took a long time to get an Iron Monger, Iron Wiplash isn't out yet whereas there are so many Iron Mans and some of them got a double release. The same for Loki from the Thor movie. A destroyer would also been great. And look at Spider-Man 3. I think Venom was for many fans far more interesting than this strange new goblin. I think DC (+Batman) villains are just more interesting to release because they were more interesting in the movies.
On the other hand for sure Green Lantern sucked and (certainly wasn't made therefore) but so did X-Men 3 and Spider-Man 3 and Iron Man 2 was also not that good but they all got a treatment nevertheless.