Those are the rules of chess.

http://www.fide.com/fide/handbook?id=124&view=article

See article 6.10:

Except where Articles 5.1 or one of the Articles 5.2 (a), (b) and (c) apply, if a player does not complete the prescribed number of moves in the allotted time, the game is lost by the player. However, the game is drawn, if the position is such that the opponent cannot checkmate the player`s king by any possible series of legal moves, even with the most unskilled counterplay.

This is draw because the opponent (you) had "insufficient material to mate"

I was playing a game, and my opponent had a queen and a king and I had only my king left.

His time ran out, but instead of me winning, the game ends in a draw the system said "Insufficient material to mate".

But that's not possible because he was able to mate with the queen and king!