I think the thing with Sterling is that he managed to keep his place in the team - not just the squad - for a long time when he wasn't playing much/well at City. Then having come back into decent form, and knowing how loyal Southgate has been to others - too numerous to mention - who've either hardly played for their clubs or played shit, it's very surprising that Sterling is continually getting the cold shoulder.
I couldn't be arsed about him personally, but, I do think that he's being treated very differently to other long serving members of what was Southgate's 1st 11.