I realize that the real answer to this question is literally just “they needed new enemies for Borderlands 2”, but let’s try to science this up, shall we?
The events of Borderlands 2 don’t take place TOO far from the events of Borderlands 1, right?
Obviously, we have Spiderants, Rakks & Skags in common.
But is there any reasonable explanation as to why Threshers, Stalkers, Varkids, and Bullymong would not be found in many of the places with similar terrain from Borderlands 1?
Oh yeah, and Scythids and Larva Works, too!
Not to mention Drifters!
Also, I see no readon why Hyperion wouldn’t have deployed Loaders to help fight all the Robo infected things.