Says who? if he was very good he wouldn't still be at Stoke.
He's the best player and captain for the 9th best team in the league. He's not really a top 4 player so is there any real incentive/opportunity for him to leave when he'd cost close over £15m?
Shawcross is the best CB we could hope to attract and exactly what our defence and team as a whole needs. As said, though, he'd cost too much for us to be in for him.