I just lost a game on time where my opponent had only a bishop (and I had a rook and two pawns). Why was I not granted a draw in that situation?
This is according to FIDE Laws of Chess.
You lost on time, so you lost.
The only exception: it is a draw when your opponent cannot checkmate you by any series of legal moves.
In this case you could promote one of your pawns to a knight and then you could be checkmated in a corner.
To avoid this kind of situation: play with increment.
Actually, your pawn on h7 with your opponent's king on f8 and Bg7# should be enough. your last move could've been a pawn or rook move, so the position is legal.