It seems to me, as is usually the case on Broadway, the Producers saw more money in their future. I don't think it would have mattered if Oak was green, the Producers got an OK from a Tony Award winning actor, who has been on a highly successful TV show for several seasons, that spells more money at the BO, in most cases. African-American, Asian, Indian or green martian, the Producers saw a way of making more money-to keep the show open, a show which is already consistently on TDF and TKTS.
I don't get that this was a racist turn at all. Now why the Producers didn't get a "name" when Groban left in the first place is a wonder.