Cost and liability are the drive behind ROV's. But the NSF panel that decided whether or not to even build another manned sub agreed with the scientists. Bottom line is, there is still great value to having a knowledgable human being doing the direct observations. Indirectly, via video feeds and so forth, has limitations too. As do trying to manipulate instruments, collect delicate objects and so forth from a vehicle on a tether.
And as someone who's been down in Alvin, and done cruises with ROVs - ROV cruises are pretty bland and boring in comparison
