It's not that they have failed as much as it is a strategic decision. ULA is not doing this by choice, they are doing it (as mentioned) to effectively subsidize the industry in Russia for other diplomatic and security reasons as opposed to technical reasons. [0]

"The [US government] asked us to buy [Russian engines] at the end of the Cold War in order to keep the Russian Rocket Scientists from ending up in North Korea and Iran," ~ tweet from ULA's CEO

ULA is in the process of replacing the russian engines as part of the shift to their Vulcan rocket and away from Atlas. I believe they have all the rockets they need in hand.

ETA: and to be explicit, they are not paying the scientists and engineers directly. They are paying the russian firms doing the work.

[0] https://www.spacedaily.com/reports/US_Rocketry_Chief_Offers_...