Communication between sub and the support ship was lost. They should have had a redundant means of location if not communication. How hard would it be to mount a location beacon on the craft to hasten the rescue?
I listened to an interview with a journalist who had been on the sub (they did not make it all the way to the Titanic) and there was little to no redundancy in many systems.
In addition to the laundry list of what could have gone wrong I am guessing they used lithium-ion batteries.
For sure! One wrench in the plan and all is lost?
Madness.