Young players can often make the transition surprisingly well, it's the same game, same tactics, same man management. Just bigger crowds, more pressure and more experienced opponents. To me, it makes more sense than appointing a number 2 like Phelan or Shakespeare. It's not a change of stroke, so much as moving to deeper water.
Guardiola at Barca, Zidane at Real, they didn't have to leave to get a chance. It's not that common, especially not in PL, but if you put in the development, they impress with youth team and know the players coming through, it can be a smooth transition, and someone unafraid of trying the youngsters they've got to know and trust.
Unsworth knows the club, knows the young players and it wouldn't be a bad thing if other managers underestimated him or don't know what to expect.