I fly this route almost exclusively, and over the past few months it has been harder and harder to get an upgrade.
I've seen two trends that are impacting u/g's:
1. Lots of -400s and -700s - there aren't as many a/c flying to Phoenix these days with 16F seats
2. More F seats reserved 7-14 days out - I don't know if these are paid F, MVPG certs, auto-upgrade fares, or mileage tickets but there are a lot of seats getting snapped up well in advance of the upgrade window(s) for lower fares.
Add in a low number of daily flights compared to other west coast cities and you have a recipe for fewer upgrades.