Why do they not make responders for underwater "listen"
In order to listen, the beacon would have to expend constant power. By pinging, it doesn't use power between pings. I would agree that a louder but less frequent ping would seem to make a lot of sense.
As for the recorders, its not so much how deep they are as how mountainous the terrain in that part of the ocean is. If it was all perfectly flat and just really deep, I think they would find it pretty easy. With terrain to deal with, debris can shift and keeping ROV's off the bottom becomes much harder. The boxes could be between 2500 and nearly 4000 meters of water in that part of the ocean. The "Jason" ROV can operate down to 6000 meters.