why stale mate?


It's not stalemate but a draw. White ran out of time and black does not have sufficient material left of the board to mate given an infinite amount of time and the best outcome in such a situation is a draw.
To win on time you have to have enough material left to potentially get mate, though the rules are a simplified version of the official ones for that (see the last paragraph):
https://support.chess.com/customer/en/portal/articles/1444798-how-do-i-claim-a-draw-