For me, the fact that DL's operations are vastly superior to UAs (my perception, but many here seem to feel the same way), and the fact that I could hit top tier Elite with my credit card waiver (as has been mentioned already, UA does not allow this), are the reasons that I have chosen to do my flying with Delta.

However, I miss the *A partner flying on FF miles. Definitely a superior group of airlines overall. But DL is superior to UA in the US.