Just the logical following of the F22 disaster: a plane so expensive that they won't ever dare endanger it in real operations... because $1 billion apiece!
I am not sure it was so disaster. The main problems from operational point of view for F-22 are operational costs (maintenance), and that it's heavy and flies like a slug. The latter is due design actually so the real problem is price.
They did not want to compromise the stealth and they wanted the plane to match the two-plane doctrine US forces have used since 2nd world war. Basically you got your agile and light fighters that clear the skies, and then you got the heavy planes with superior firepower that mop up the rest. So yeah, that's what they got.
F-35 attempts to be made from cheaper parts and with cheaper costs, and lighter. That means a lot of pea counting, and using simply worse parts. No wonder that program has had problems.
Operationally, F-35 is barely stealthy, and it doesn't fight that well. I'd rather sit in the cockpit of F-22 at this point.
