My guess is that Geno wanted Graham more and sees him as a great offensive mind. Geno obviously knows Armon Gates and probably wanted him as a backup candidate. Armon seems to have some great ties to recruiting, especially in Chicago. I feel he would have been a better hire from a recruiting standpoint. Graham appears to be a better X's and O's guy.
My hunch is that BU had interest, but Gates decided to jump on the "sure thing".