I guess it might just be because he is your quiet, calm, tidy, Barzagli-type centre-half, rather than being one who is going to go getting a lot of attention or grabbing a lot of headlines.
He's also only had the chance to prove himself at Udinese when playing in a back three, so perhaps foreign sides aren't sure he can play in a back three.
Anyway, if he goes for about ~8m, I'd say that's a reasonably fair valuation.