why is this a draw with insufficient material?

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Avatar of JumboKing66
https://www.chess.com/live/game/121819064116
Avatar of Martin_Stahl

Black tan out of time and can't win. White only has a king and can never checkmate with a lone king, so it's a draw by insufficient material.

https://support.chess.com/en/articles/8557986-my-opponent-ran-out-of-time-why-was-it-a-draw