Firstly because we always had better options. Secondly because he does walk batters, which is a problem -- although not the death knell you make it out to be. Thirdly because he doesn't strike people out, which is considered a prerequisite for late inning work with surprisingly few exceptions.
I personally believe Lopez was misused in Boston. Not because he should be a setup man but because he was given the chance to clean up after himself much too infrequently. A guy who gets as many ground balls as Lopez does can usually work around a walk or if his defense is good and he's a bit lucky, even turn it to his advantage with a ground ball double play.
Don't presume to tell me what I think.
Honestly? Each of the 3 years he spent in Boston there were relievers like Gagne, Timlin, Hansen, Seanez and Tavarez who scared me far more than Lopez did. I'll take messy and effective over messy and ineffective.
It's only this last year where the rest of the bullpen was so loaded and Lopez was so rough out of the gate that I even thought a roster move might be necessary. And frankly, if we weren't so deep and had to stick with Lopez he'd probably bounce back given half a chance.
You're right -- JC Romero never had as good an ERA for 3 consecutive years as Javier Lopez has for 06-08