ULA's vehicles were designed around the RD-180 rocket engine [1][2]. This is an engine built and designed in Russia.
After Russia annexed Crimea, Congress banned the Pentagon from using Russian rocket engines. Russia responded by counter-banning the Pentagon from using its engines [3]. This made things complicated for ULA.
Or if you don't have a reliable supplier of engines. In this case world politics meddled with space rocketry, so supplier's goods became less than completely available.
Only Atlas-V. Delta-IV uses American hydrogen engines (RS-68); however Delta-IV is rather more expensive than Atlas-V. Even though Atlas-IV is rather more expensive than Falcon-9 - which uses simpler, less performant but cheaper technology.