My take is that this is simpler than people are making out. " Circumstances conspire and combine" is not reality. Conditions only exist. They have no consicousness or means of combining against you. We allow them to " conspire" by ignoring them individually and forgetting what the one job of a pilot is. That is to fly the aircraft to the point of a safe landing. Ignore Kobe if he calls on the ics when you're busy. In fact isolate the passengers if you like, if they are chatty when you are sweating. No need to dial in that squawk code if you are busy actually flying. It can all wait, it isn't going to save your butt. If you've ever lapsed from your task of driving a car due to a phone call, text or something that just draws your eye off the road then you must know this is no different. He obviously had the training and ability to revert to basic instrument flight ( had the rating and had UA recovery training) but chose not to focus on that. What he apparently did not have was the judgement to see that in a little while all would be forgiven if he landed short, in a vacant parking lot and called cabs for the passengers. I suspect that his thoughts during the flight, until g forces were not computing, was " I can get through this" , not " I need to be on the ground right now, if not a few minutes ago".
The words I rember most clearly from my ( airplane) instrument instructor were " Quit fu*king with the radio and just fly".