You don't trade your franchise LT away when you need to keep whatever QB prospect you have (Rosen, Tua, Lawrence, whoever they settle on) from dying halfway into his rookie season. It's what partially prevented Tannehill from being better than what he was and has been a sticking point on this team for a damn decade.
Tunsil was only 25. OL have shown they can play at an elite level well into their 30's. You keep him as a building block and to protect whatever this team's future is's blindside.
Edit: Not to mention the supposed "revolt" that was coming if the Dolphins traded Tunsil. Players don't want to play for losers, and this move signifies that is what the Dolphins are, as much as it needs to be done.