Because all together, it can make a lot of time ( A new player must be found/matchmaking and the new player has to arrive/load the map). It could be kind of a incitement to stay (...) and be more fair for the unstaffed team. Just unfunny to arrive when every body is cracking his super, or you have to wait the end of the party to maybe have one. AND when we join a game in progress, could we at least know during the loading screen which map and mod ? Sometimes, the landing is brutal (saving private Ryan - the Normandy landing scene brutal...) ;
Add penalty to quitting in Quickplay. That'll make the quitters not quit, but you can bet more than half the crucible players would be banned immediately. With Supremacy making a return into the Classic Mix playlist, everyone would be banned for a Supremacy match.