Asked if it was a difficult decision to make, the goalkeeper said: "It was and it wasn't. Chelsea is a fantastic club, it would attract any player and it certainly attracted me.
"It is a fantastic place to play and they are the current Champions League winners. They are the European champions.
"But Stoke are a fantastic club for different reasons and somewhere I feel I can better develop my game at the minute. It is as simple as that.
"Chelsea didn't do anything wrong, I just didn't feel it was the right move for me and felt Stoke was.
"It was a very, very important decision to make about when and where to move. It took a lot of thinking and unfortunately Birmingham were in the position they were.
"I have been there for a long time, learned a lot there. I've had my spells in League Two and my taste of the Championship - but I feel that if I really want to push Joe Hart and the senior keepers for the senior spot that I need to be playing Premier League football.
"I feel capable of doing it and feel capable of doing it with Stoke. It is a very good set-up there."