Likely because people are emboldened to action when they see themselves in those who are being unjustly treated. It's no secret that Silicon Valley is not terribly representative of black people, veterans, and so on. If this is a fault of the folks at YC, I'd wager it's a fault of many many other people as well--so while I feel it is valuable to point out that issues, I don't think it's constructive at all to point at a good deed and ask, "Why not sooner?"