There are only 30 teams in the NBA. If you want to be a head coach, now or eventually, you have to get after whatever more senior position is open. Loyalty ain't got nothing to do with it.
I don't know if you saw in the broadcast of Game 1, but Blatt was almost one of Kerr's assistant coaches: got offered the job, took the job, but then the Cavaliers came in at the 11th hour and offered him a head coaching gig. Kerr gave him his blessing, and here we are.