Andrea D’Amico the agent of Criscito spoke to Sky Sport about how his player was close to Italian clubs in January:
”Inter and Napoli asked for Criscito in January but the economic and sporting conditions were not right so it was not possible to close the deal. It was not possible to equalize the offer from Zenit which made us decide to continue with the Russian club.”
Source: Sky sport

Why we have Santon already?
Santon was a last minute panic buy. I suspect we inquired about Criscito prior.
And in case you haven’t noticed, 1 quality full back is far from enough. Criscito or Darmian would be astute buys.
I think his agent explained why Criscito chose to extend his contract until 2018… so there are little chances we get him, he only cares about big salary. There must be better options out there!