1. E+ for family as Plat/1K
2. Star Alliance means large network and a LH domestic lounge once in a while.
3. UA's miles have value
4. SDC . . . you don't appreciate it until you need it!
5. 1K bennies like Global Entry, GPUs/RPUs, and 100% mileage bonus
Their route network is best out of DEN, but I could always fly a competitor, especially if I'm willing to make a stop. So the above are the reasons.