Here's a lede from a recent Variety article:
There's an ironically presented black minstrel show at the Vineyard Theater. There's a darkly comic wrestling match with retro racial stereotypes at the Public Theater. At Playwrights Horizons, white characters and black characters try to talk about race, and mostly fail.
See a pattern?You know what? No! I don't!
Of course the "pattern" allegedly is: guess what, we have a black president now!
To which I respond, guess what, there were indeed plays about black people before 2008! There were even plays BY black authors before Barack Obama was born.
Seems to me that even if the case is that such work is now getting done more in NYC, that's pretty specious, too. I don't see any stats in this piece, but I bet back in the 70s, for instance, there were seasons of even more African American work on Broadway let alone on the fringe.
As if the argument isn't empty enough, having as your only sources the three white AD's of the theatres you're talking--who, funny, seem to totally agree with you that they are civil rights trailblazers--doesn't help.