I doubt its cap related. More likely what SGP said - Leeds just don't feel he's the future star he's been touted as. Lets be honest, if he was showing that kind of promise he wouldn't be leaving, or if he did it wouldn't be to York. He doesn't look strong enough for the centres, and would be hopelessly outgunned in the backrow, which he was also said to be able to play. He may just be a case of outstanding junior who doesn't transition to first team.
Smith clearly likes physical presence in the centres, and he's signed MacDonald, Roberts and potentially Olpherts to play there. Simpson is a year older and if he's fit he'll be in the mix as well (although Smith hinted strongly last year that he didn't like picking teenagers).
Also someone mentioned Corey Hall - he left before Smith took over.