My thought as an amateur here are that UA is charging more for the desirable itins because more people have booked them. I don't think it's anything nefarious.

Not that you've asked but AA has done this for many years on their awards. For example, I can get a 20k first class trip to Cabo if I double connect and overnight in PHX. For 6 hours! If I want a sensible itin, it'll cost my 90k. UA is not quite so literal with the cash to miles ratio but they are definitely trending toward 1 cpm unfortunately! I still don't quite understand how the pricing engine determines award fares and my friend has not been forthcoming. Hoping to jump him at the next meetup if we ever get one!!!