For whatever reasons, it seems like the standard upper stage engine in the U.S. inventory is the RL-10. A high performance engine with nice specific impulse (Isp = 450-465 s), but very low thrust (25 Klbs).
SSME is a great performer in Isp (452 seconds in vacuum and 366 seconds at sea-level) and much higher thrust (512 KLbs thrust), but that is literally 20 times the thrust.
Due to the difficulty in air starting (really starting period) an SSME, they changed to the notion of using a J-2X with good Isp (448 s in vacuum) and about half the thrust (294 Klbs).
So a 20 fold increase in thrust seems like it would be more of a hindrance than help. J-2X with only a 10-fold increase in thrust seems like it would be problematic as well.
You can throttle all you want, but a 10-fold or a 20-fold thrust increase seems like a big deal to accommodate.
I understand that Isp is king in upper stages, which is why the fairly low thrust RL10 is so effective (similar to the discussion in the question How is an RD-180 engine a reasonable replacement for 2 NK-33/AJ-26 engines on Antares?).