Southgate is obsessed with keeping things as unchanged as possible I think - that's how he thinks you win tournaments. Very minor changes, maximum understanding between the players, group that 'likes' each other and a loyalty to those who play well for the national team (which by and large Maguire has, as much as he has become a bit of a joke for his club side).
I'm not sure if it's the right thing to do or not - clearly Maguire would not be in on form but would be a big call to play Ben White or someone having not properly blooded other CBs in the run up the tournament. Southgate will live and die by it I think this time. Unfortunate that someone like Maguire is so out of form.