You can't drag 1km of tough power and control cable behind a little robot, and autonomous robots aren't up to the job yet.

This thing has a 2km long fibre optic tether and is battery powered. Been in use since 2001. https://en.wikipedia.org/wiki/Seafox_drone

Cool. But that vehicle is open underwater only. These caves are a mixture of swimming, walking and climbing. Lots of power required, and very tough to teleoperate.

Because robots up to the task don't exist yet (and aren't even particularly close). It's a very hard problem.

It has nothing to do with it... Robots are not an option because it's a zero visibility environment (cameras for remote operation are useless).

I'm happy to stand corrected. However, zero visual visibility doesn't seem like a limitation for a remotely piloted robot. Can't they just as easily use sonar, radar, or some other form of non-optical "vision"? Fishermen use "fish finders" which are non-optical, so I'd guess the tech is pervasive and cheap.

I don't expect sonar would work well in turbulent water, with highly reflective (rock) jagged cave walls.