Gascon said not all witnesses had been questioned, and no independent witnesses had been interviewed in the stabbing.
"It is vital for our office to have independent corroboration of the incident in order to meet our ethical obligation to charge this case," Gascon said.
San Francisco police did not immediately respond to messages left seeking comment.
Police Chief Greg Suhr said earlier that Montgomery made "incriminating statements" that led to his booking the night after the stabbing.
Suhr said Denver, 24, was with his father, older brother and two other people a few blocks from the Giants' ballpark when they exchanged words with some Giants fans.
"The back and forth, 'Go Dodgers!' 'Go Giants!'" Suhr said. "And it deteriorated from there."
Montgomery's father told the Lodi New-Sentinel his son was jumped during the fight, and he stabbed Denver in self-defense after Denver and others yelled "Giants suck" and hit his son and friends without warning.
A second suspect was questioned and released by police earlier Friday.
The stabbing was the latest in years of incidents stemming from one of the most passionate rivalries in American sports.
Two years ago, Giants fan Bryan Stow suffered permanent brain damage when he was attacked in Los Angeles.
There was a moment of silence for Denver before the Dodgers hosted the Colorado Rockies on Friday night.
"I just can't understand how, sporting event aside, society's gotten like this," Giants general manager Brian Sabean said Friday. "It's bizarre to me."