While the Riberios are legends in BJJ, they're not exactly to par with MMA training, and (as someone else pointed out) cornering. BJJ in MMA and normal BJJ are two different things, and I think that's mainly why Diego had to leave.
While I don't particularly like Jackson's gameplanning, he does know how to win fights and is a great corner man. At the end of the day, a wins a win and a loss is a loss, and when you're an MMA fighter the way to pay the bills is getting a win.