Because when it comes to Tammy vs Wilson it's purely down to form. Neither set world alight for England in games they have played, and they are probably not gonna play anyway. So go with most inform.
But certain situations you have to weigh up form vs other things. That's what he's done with Maguire and Philips. Maguire is the big call and Southgate's job will probably depend on whether he performs. Ala Sterling last Euros