Sterjovski, 30, has signed a three-year deal with the Australian A-League side as he aims to secure regular first-team football and make the Socceroos squad for South Africa in 2010.
"The problem was I wasn't starting enough," Sterjovski, who made 30 appearances in 18 months at Pride Park, told the Derby Telegraph.
"I wasn't playing enough games and that's going to be important for me in the next season.
"I had a great relationship with everyone at the club, I loved Derby and the supporters. There were no problems there at all.
"But I'm determined to be part of the World Cup and playing regularly at a successful club will enhance my chances."