ULA's reasoning was pretty straightforward. The main reason they dropped out of the competition is because the bidding was to be decided largely on price, and they can't compete with SpaceX on price -- their selling point these days is that they're the boring, less-risky option that nobody ever got fired for buying. This is totally a valid selling point, until SpaceX gets a longer track record, but GPS satellites are the kind of payload that wouldn't be too disastrous to lose, so in this case going for the cheaper launch option makes a lot of sense.