My suspicion is ULA can’t manufacture SLS quickly enough, at high enough quality, to meet multiple, gradual tests.

ULA has nothing to do with SLS.

It's everything. NASA doesn't have the money, brainpower, efficiency etc. to implement SpaceX development method. They can't fab it fast enough, nor can they iterate on the engineering fast enough, nor are they will to sustain the optics of a "government rocket blowing up" like Musk is. They don't have the caliber of engineering talent available or a workflow setup (high autonomy, long hours, better pay).