The situational awareness of an ROV operator (who can only get visual feedback (in very low visibility conditions) is far lower than a diver (Who can feel their way along). Way too easy for an ROV (which has to tow along a tether) to get itself tangled somewhere.
Is sonar an option at the kind of scales involved?
I am by no means a subject matter expert, but I'd expect sonar would be next to useless in turbulent water, close to a bunch of highly reflective surfaces. Sonar also doesn't solve the problem of a kilometer long tether getting tangled at some point.
Visibility is basically zero, a remote operator wouldn't be able to see anything.