Barr had the resume and said the right things.

Former AG, so obviously qualified, and getting senate republicans on board will be easier than elevating someone with no experience.

Supported (or at least he said he supported) Trump firing Comey, criticized the Mueller investigation, questioned why the DoJ wasn't investigating Uranium One...

With hindsight we now know better, but I can see why someone would think he was a good pick at the time.