It's the routes ...
I fly out of EWR and United goes everywhere I have customers. (A domestic air warrior):
Houston
San Francisco
Tampa
Chicago
Atlanta
Boston
Colorado Springs
Dallas
Las Vegas
+ lots of other 'less common' trips ...
4) The staff. United always takes excellent care of me. I never worry about being stranded.
3) Economy Plus
4) EUA
5) did I mention the staff? I know United's always going to get me home.