Bush 1 lost because he flamboyantly lied to the American People.
It really is that basic.
If you mean the "no new taxes" pledge, I disagree.
We would disagree on whether he intentionally lied at that time. My position is: there's no way to know. He may have been sincere, or he may have just thought it was a popular campaign slogan.
But, it was a clear campaign promise. Bush broke it. He claimed to have good reason for it, but he didn't articulate it in a convincing way.
And like every promise to cut spending the Democrats made in exchange for raising taxes, that one was broken too. It wasn't the first time, and it wasn't the last. Both Bushes never learned: they were the archetypal Charlie Brown Republican that landed on his butt when the Lucy Democrats pulled away the football.