And of course there's a difference between actually going somewhere and sending a robot there instead of yourself - "to explore" infers a physical presence, rather than passive/active watching. Sending a robot to Mars hasn't quite the same impact as sending a man (although it's still an amazing feat and deserves respect and congratulations). Looking at satellite images of the top of Everest is another example etc..
Also, although ROVs can achieve much beyond the physical limits of humans they probably (I admit I'm guessing on this one) cost a shed load more money than fully equipping yourself with diving gear - and who the hell is going to provide insurance cover for using an ROV in a cave in case it gets stuck?
OK, so Wookey 26 goes to -60m; technical divers are able to dive beyond that (at least to -100m+)... it's just that the way on tightens too much (according to the superb film footage of Gavin Newman's "Wookey" film... if you haven't seen it yet visit Hidden Earth 03 and get an eyeful.
However, if you invent an ROV that can voyage to the parts of caverns that other cavers cannot reach I'm sure there's a pint or two in it for you!... perhaps you'll discover caverns measureless to ROVs.