This rule is in place to prevent “queue dodging” which is an act to avoid the match if you don’t like a certain player, map chosen, or the team composition. Furthermore, penalizing disconnections and crashes help deter players from attempting to play Competitive and instead focus on resolving their technical problem. Once the match cancellation sequence triggers, a 10-second timer is displayed to simply alert remaining players the match will shut down (as a courtesy to them, not for the player who crashed).
Details of these policies can be found here:
If you need help troubleshooting your crash, please review these starting troubleshooting steps or start a new post in the technical discussion forum for further assistance.