Why is this a stalemate and not a checkmate?

http://www.chess.com/livechess/game?id=650692790

Because there's no check.

If the king isn't under attack it's not checkmate.

Stalemate is when it's your turn but there are no legal moves (and the king isn't attacked).

Black isn't in check and has no legal move. That's the definition of a stalemate.

I understand. Thank you.

In your settings go to 'Stalemate Rule' and instead of 'Chess' select 'Shatranj'. From then on positions like the OP will be counted as a win.

Not in chess.

If you play by the Shatranj rule taking all the pieces of the opponent also counts as a win. If after your last capture the opponent king on its next move can also take your last remaining piece then it is a draw ( not in Medina though where it is still a win ).