Defoe, who finished 10 goals behind Tottenham Hotspur's Harry Kane and nine behind Leicester City's Jamie Vardy in the race for the golden boot, enjoyed a productive season, scoring 15 league goals and helping to steer Sunderland to safety.
England manager Roy Hodgson, however, chose youth over experience in his 26-man squad for the European Championship on Monday, including 18-year-old Manchester United striker Marcus Rashford at the expense of the 33-year-old Defoe.
"Obviously I've not been involved with England for quite a while and I've always thought that getting into an England squad before a tournament is based on merit," Defoe told British media. "I just feel like I've done everything that I could do.
"As a forward if you score goals then you've got a chance of going. So obviously, when you're not in the squad, it's a disappointment."
England face Russia, Slovakia and Wales in Group B of the 24-team tournament in France, which starts on June 10.