I can see the game you are talking about, don't take it as a personal slant.
Your opponent played on till checkmate in 565 of the 1000+ games he has lost.
Some players, as is their right, play on till checkmate. He also has 120+ other
games on the go so maybe he has not got around to replying to you yet
Either he's got too many concurrent games going to reply, or he's hoping you'll die before the timeout expires. Look both ways before crossing the street!
As you know, there are three ways to lose in chess: 1. Checkmate; 2. Time; and 3. Resign. So, by the way you are presenting that it is unsportsmanlike conduct, which the player is not, he can chose to lose on time. It is interesting how some make up rules to fit their needs of winning.