Because people care way too much about their solo shuffle score. There's an easy way to game the system by leaving because you only lose CR but keep the same MMR which is the most important thing about climbing in shuffle. If you lose a lot of MMR, to climb back up you will need to do very well your next games. If you just lose CR you can probably go even your next few games and still gain rating because your mmr will be higher than CR.

Mes talked was talking about it yesterday in a 2500 mmr game where a rogue that was getting crushed left his game. He said that was probably the best option if you think about it from the rogues point of view even though it's fucked up. This was also something people even in the prepatch were abusing for a fake seasons numbers.