Why does NASA require the help of ULA, and if they don't, then why do they use them for launches

I mean, ULA charges enormous amounts for a launch, wouldn't it be cheaper for NASA to just launch the rockets themselves?

And if not, why don't they go to some of the cheaper alternatives like SpaceX to get Falcon heavy launches for the bigger loads that Falcon 9 can't muster. (also I know that NASA already uses SpaceX for supply deliveries, but I just don't get why they don't use them more often as they are obviously the cheapest choice).

also, how did ULA acquire the rights to launch NASA developed rockets like Atlas?

Submitted November 20, 2018 at 03:00AM by Buihelgason https://ift.tt/2DxabHM