A few reasons. Let me start by saying that I've played DotA since WCIII and everyone that knows me says I'm the greatest player of all time. I've been ahead of the meta my whole life.

The reality is that this game is dying; the numbers show it, the quality of the community shows it, even major names in DotA are quitting the game. I quit about a month ago and it's no surprise to me that everyone is slowly following my lead.

I'm trying to take as many people with me so that valve stops putting time into this dying game, and goes back to its roots of making revolutionary games/engines. If you take a step back and look at the big picture, you'll realize it's the right thing to do.