The Welshman made 11 appearances for the Robins during a successful loan spell last term, but opted against a permanent switch to the County Ground after penning a three-year deal at Carrow Road earlier this week.
"I will be completely honest with you - I'm very disappointed because we knew we were down to the last two (clubs)," Fitton told the Swindon Advertiser. "Owain is a good player who we gave the chance to come back and play, which was important to him.
"He aspired to go to a Championship club and we thought we were out of the game. But we held talks with Owain and his advisor and we know it came down to two clubs."
Meanwhile, manager Danny Wilson spoke of his delight after Craig Easton, Phil Smith and Lescinel Jean-Francois all agreed new one-year deals with the club.
Wilson said: "It looks like they have confirmed, which is great from my point of view. The quicker we start to bulk the squad up the better in preparation for the tough start of the season we've got in front of us. It's nice to get the boys on board."