in short, McDonnell-Turning-Into-Boeing saw dollar signs during the late 1990s as the dotcom bubble tech boom started manifesting in "more potential satelllites to launch" and then built a rocket that was not financially viable except at an absurd flight rate, using it as their bid for the EELV program to get defense funding and contracts for it.
The dotcom bubble bursts almost before it even starts flying, and they were left holding the bag with this big orange bastard of a rocket that flew on average like twice a year and was never sustainable to operate, given it was supposed to fly like, forty times a year. That's the scale they built for.
It was trying to be the falcon 9 of its day but with McD and Boeing's greed building something that wasn't efficient except at a massive scale, vs Falcon 9 being designed to be cost-effective at a minimal scale.
There was also the extreme amount of corporate espionage with competitor Lockheed and their Atlas V but that's a story for another day. those two combined, and Boeing threatened to quit space altogether, so the federal government broke out the Get Along Shirt and ULA was born to manage both Lockheed's Atlas V and Boeing's Delta IV so the department of defense could have its "assured access to space" (redundancy)
essentially,