Because an implosion is a single point of failure leading to instant death. Anything else can be salvaged by staying calm and using one of multiple ways to resurface.
Subs will always have 1000 ways to kill you and most of them are catastrophic. You're pretty much boned if anything goes wrong especially with so few crew members, lack of compartments, backup propulsion, etc.
Really? Anything else can just be magically salvaged? I'm not sure there is that much room for error 2 miles below the surface
They are visiting a debris field. There is a much more than zero chance they are stuck on something.