A good reason was because "it sucks and we suck because we had seven long years to come up with a bulletproof replacement and failed. And even though I was part of Congress all those seven years, I never added anything significantly altering to make said replacement bulletproof."
There's the problem. It's not like he was pulling a Rand Paul and screaming about it months and years in advance. Nor advancing his own changes to be made. He went with the crowd and when it came time for the decisions to be made, all the sudden voted against the replacement.
Like I said, I don't hold that against him. However, I do question why.