Speaking as a Texan, I do not recall seeing any evidence that, in his former role as Attorney General for Texas, he was serious about reigning in prosecutorial abuse. He didn't really make any effort to reduce the prosecutorial overreach and over-imprisonment perpetrated by his own office when he had that opportunity, and he hasn't made any effort at reform in the Senate, either. If anything, his main efforts over the years have been in the opposite direction: to imprison more people, for longer sentences, with fewer procedural safeguards and less court oversight.
I blame the "tough on crime" prosecutors and politicians (and the people who vote for them) for a large part of our current mess.
You weren't paying attention. Cornyn was a judicial system reformer in Texas as both a judge and as AG. Open government and fighting good ol' boy corruption were his beat.
By judicial system reform, I mean reducing the US's fetish for throwing too many people in jail for too-long sentences, which is what's relevant to the present case. I don't see any evidence that Cornyn did anything to reduce Texas prison populations or direct his office to stop intimidating defendants into accepting plea-bargains, or to stop seeking excessive sentences.
I blame the "tough on crime" prosecutors and politicians (and the people who vote for them) for a large part of our current mess.