Because of the prevailing infrastructure. If instead he had created a customized boarding school for the inner city kids somewhere safe and pleasant we'd be seeing real meaingingful change in their outlook right now.
As it is, they were in the same unhealthy environment they've always been in. It's not the schools, it's the teachers, the streets, their peers, the home life, everything.
A boarding school is the best solution to fix all that.
What are referred to as "inner city schools" can more relevantly be referred to as re-segregated schools or just never-integrated schools.
ProPublica came out with extremely compelling data on this subject just several weeks ago.
http://www.propublica.org/podcast/item/how-5-florida-schools...
It's a bit thin on data / statistics..
Hopefully Zuckerberg learned a valuable lesson there -- improving inner city schools is simply not contingent upon how much money is thrown at them.
Case-in-point: http://www.cato.org/pubs/pas/pa-298.html (warning, long read)
Are we really going to begrudge Zuckerberg for trusting Cory Booker, who was by all accounts very persuasive, and investing $100M+ in inner city schools? That's hardly bad behavior as the article seems to imply.