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.