Jason Davis has a good rundown on it, and the implications for Europa Clipper. I don’t know how he knows this, though:
Any other rocket besides SLS—including SpaceX’s upcoming Falcon Heavy—lacks the power to blast Clipper directly from Earth to Jupiter. A conventional rocket would rely on three gravity assists from Earth and one from Venus, increasing the transit time from about 2.7 years to 7.5 years.
How does he know that? Has he run the numbers, or is he just taking NASA’s word for it? He’s also not considering the possibility of New Glenn, New Armstrong, Vulcan/ACES with a distributed launch, or BFR, all of which could be ready by 2022.