The Bills didn't want a rookie at LG, otherwise they probably would have started Preston (which I HOPE happens now). So it was either take a chance on Big Mike and hope that at least he shows something, or stick with Anderson. So after Mike showed he couldn't hack it, it was back to Big Ben. I don't think keeping Big Mike in there would have helped things any, if not made things worse.
This has been rehashed time and time again. It seemed like a good pick at the time. Personally I think the problem lies with Big Mike's motivation and toughness more than anything, and as they say, you can't measure heart and desire.