widgeon, that's a good question.
I think part of the answer lies on large muscle movement logistic and fleet support considerations, and thus program cost. Romeo preceded UH-60X which became UH-60M by a few years. At the time of the decisions on engines, (before the year 2000) keeping the T700 was one way to keep program costs down. (During the Clinton admin, there wasn't much money to spare on any program ... different topic).
T700 was the common engine for, from NAVAIR's point of view, Cobra and seahawk, which by that point were B, F, S, and now R models.
It may be that with the UH-60M being brought into and beyond IOC (200 or so have been delivered to the Army so far), Navy may re-look their Romeo engine and perhaps make the step to put a little more power up there. That would probably happen during depot/SDLM cycle, if ever, and take a decade to implement.
Not sure what CG/weight/load/airframe mods would be needed to do that, or if it's even cost effective to do so.
EDITED: I think the Navy would rename that engine "401D" if they got it, since they tend to use "401" where the Army uses "701" for reasons I do not recall. I had it explained to me some years ago, but I've lost that bit of memory.