Everytime that gets brought up, I get reminded that ULA isnits own entity, not the same people working on SLS blah blah blah...
The real answer is two fold imo:
First, the shuttle launch infrastructure, as NASA pointed out in their hour long excuse making session, was a finicky as the rest of the system, and missions routinely got scrubbed because of the problems. The senate launch system took all the finicky and unreliable parts of the shuttle, stuffed them in to a bizarre amalgamation, and put it on the pad with insufficient testing and improvement.
Second, ULA( and it's various members) don't get payed more money when they delay or **** up a launch. Boeing et al just get more money everytime they eff this thing up...why would they bother to improve things when they can just keep milking the cow. I think they've realized that SLS will not survive as-is beyond the missions that have engines, so they're going to drag this out as long as possible.