Might anyone have a view under the hood here, and not be under NDA, to comment on why ULA hasn't replaced Northrop Grumman with a less failure-prone SRB supplier?

I can't think of any other remaining large solid rocket manufacturer in the USA, NG bought them all.

They would have to admit that they were wrong to switch away from using the proven Aerojet-Rocketdyne AJ-60A boosters (developed for and still flying with the Atlas V family).