In a statement emailed by one of his agent's employees, Harper said: "I have attended NatsFest each year and always enjoy my experience with the fans, but was unable to attend this year's event due to matters out of my control. I look forward to next year's NatsFest."
The Major League Baseball Players Association has filed a grievance on Harper's behalf over whether he should be able to void the 2015 terms in the final season of a $9.9 million, five-year contract he signed when drafted. An arbitrator hearing could happen Tuesday if the case isn't settled.
Harper is represented by Scott Boras.
"We're disappointed he's not here, but he chose not to be here because of the grievance," Rizzo said.
The GM said he couldn't discuss the grievance because it's "a legal matter."
Outfielder Jayson Werth met with fans Saturday but, unlike other players present, did not speak to the media.
"That was my decision," Rizzo said.
Werth was convicted this month of reckless driving and sentenced to 10 days in jail.
The Associated Press contributed to this report.