Simple, Hollywood is quickly running out of ideas so they need to churn out sequels to every money maker they have. Few of these films are actually worth revisiting. This one included.
Instead of trying to rehash the events that made the first film so great, they go the direct sequel route. Boring and predictable.
It's not exactly exciting. It may not be a rehash, but that doesn't make it any less unnecessary. The cleverness of the first film has washed away, and we're left with a bad cop drama for the first half, and bland thriller the last half. It's all got a been-there-done-that feel to it.
Not worth watching at all.