When planes are in trouble, sending out maydays are the last thing the pilot is worried about. They are panicking trying to save the plane.
The Buffalo commuter plane was only flying at 1600 feet when the situation occurred. A nose dive from that altitude doesn't leave much time for anything. This plane was likely well over 30,000 feet, and if it were nothing short of catastrophic, it would have been a ditch scenario - in which case the pilot/copilot would have certainly sent a mayday.
Yes.....
.....either the pilot(s) had no time or there was no way to send a distress signal because the radio was dead.