Sure. But.
Marinovic - established himself as the No 1 last June at the ONC. Not being dropped. (I wonder, if the AWs had had a March 2016 game, and if Moss had been fit in May 2016, would Moss be the No 1 now?)
Williams - always been there w/o getting any game time. Rewarded for loyalty. The one to drop out.
Moss - Hudson said he was No 1 at his first squad naming, a range of reasons meant he didn't get called up until last November. But having given him that backing then, and Moss having hung around, and not said stuff it, he gets rewarded.
So there are compelling reasons for all of them to stay ahead of Gleeson - who was a victim of the whole Olympics/under-23 thing for large parts. I can't recall if his missing the ONC was ever explained. Whatever.
I don't think Gleeson is vastly superior to any of these others, the way Wood is over any of our other strikers, or Reid is over any of our other centre backs. So 'non footballing reasons' do a play a part, and don't necessarily need to be nefarious. Marinovic and Williams are regular members of the squad and the environment, and Moss has come in as the de facto (but no longer) No 1.