The US military heard it and didn’t necessarily want to give away capability of listening devices around the sea floor.
The sub was difficult to get to the debris field because it was at an incredibly deep section that few craft are capable of reaching safely.

It was frustrating they made a big deal about something we ultimately could have done nothing about in the first place. However it’s not like the whole “hearing the implosion” thing was something the military wanted to give away and at that depth we have to be careful. Don’t forget we’ve put more people into space than have been to the deepest point on the planet.