Martin Scorsese is a phenomenal film director, and he's achieved everything within the field. However, back in 1969 he was very young, and not as adept as he would go on to be. He also physically couldn't be everywhere at the same time. If you were on the main stage at the same time he was filming, your performance made it onto his documentary, and your profile was boosted worldwide. If you weren't, then even if you were a major act at the time, it's likely that most people don't even know you played Woodstock. That has to be frustrating.
Different performers found themselves impacted by this in different ways. For Joan Baez, the woman in the picture, she became an icon of the festival and forever linked with the event in many people's minds. For Sly and the Family Stone, however, it was as if they'd never played at all even though they were a massive act during the era.