It's pretty simple: your opponent times out, so he doesn't get an opportunity to play on (so, no chance to mate you). If you have any chance of mating him (on the board, given you have not run out of time), you'd be given the win. But because you don't have any chance of mating him - as you have "insufficient material"* - you don't get to win either.
This is considered a drawing condition in chess.
Think of it as: if the game ends because your opponent times out, the result is the most optimistic outcome possible for you given your position on the board. If you can say "this and this might have happened" and led to you winning, then you win. You cannot argue that here as no move sequence delivers mate with a lone King. The best possible outcome for you had the game continued is still a draw.
*Strictly speaking FIDE (unlike chess servers which need a simple heuristic like material count to go on) does not just do a material count to determine the winner. The question to be answered is "is there any sequence of moves by which a mate can be delivered by player X?" and this is answered in the problemists' style, i.e. with no regard for how reasonable it is to expect player Y to play along with that sequence. But in practice it doesn't matter here since with a lone King you (player X) obviously cannot mate your opponent (player Y).
