It was probably done purely out of contempt/spite.
Gen Con has always hated Bigdill23. And now it looks like they've finally screwed him over for the last time. Why, Gen Con? Why?
But, seriously, it's got to be incredibly complex to schedule. Unlike most events, which just need 1 hall at the convention center, GenCon needs everything. Plus Lucas Oil. Plus all the rooms in the surrounding hotels. It's amazing it can come together at all. I've been going for over 3 decades. I used to think of it usually as mid August, but it was entirely in July for the first one Indy -- and as late as August 25 waaay back in the 1980s. (I may be dating myself.) Point being, it is when they can make it work.
I'm puzzled by the comments about school starting. Someone said it was the first day of school? Or after school started? Does school start August 2nd in your area? If so, even if that is Indy, you shouldn't assume they will plan around that. GenCon draws from around the world, not just the local area. I'd expect that in the US, most schools don't start until Labor Day or thereabouts. August 2nd seems like a whole month early compared to where I live.