Because after the age of about 12 what team you support doesn't really matter mate if your a professional footballer
City are in much better shape over the past 3 years than United are
City have the better manager - especially so for someone with Stones style as a defender
City have more money
City are much more likely to win things than United
City will play in the CL next season not the EL like United
Also a lad from Barnsley may be a huge United fan, but he won't have that same sense as a lad from Manchester would have about the derby rivalry etc anyway, and throw in what i said about it not really mattering for footy players who they support - unles ofc your talking about Liverpool in which case they almost demand that anyone they sign has always been a boyhood red
City may have the money, but they have don where near the worldwide popularity that Utd have. Whether Guardiola is a better manager is open to debate, and based on being a centre-half needing a mentor I'd be as bold enough to say Koeman will provide better guidance than Guardiola.
Utd under Jose are as likely as City to win things.
And that stuff about not being from Manchester and so not having the same sense of rivalry is just hogwash I'm afraid....I'm a Manc, been an STH for nigh on 40 years at GP and if you think I don't dislike the [Poor language removed] like someone from L 4 then you're very misguided.
And, he's seen how great the big City signings are first hand.....Raheem Strerling for one!