The 26-year-old completed his switch to the Championship side last week after negotiations with the Magpies broke down towards the end of last season.
Simpson told reporters: "There weren't any arguments. It's football – these things happen.
"The fans need to know I wasn't being greedy. It was just one of those things. I think the club wanted to go in a different direction. I knew that even when they offered the contract."
Simpson made 115 appearances for Newcastle in his three years at the club.