Or, if this seems too simple an explantion, maybe they just wouldn’t pay her to do them.

BUFFY DVDs Lack Gellar Commentaries
It’s not lack of interest but time constraints that prevented the BUFFY star from doing episode commentaries on the BUFFY DVDs.
As she explained to Sci Fi Wire, commentaries were done while the stars where filming BUFFY episodes. “I was working constantly, three units. There was never time. It was unfortunate. It was a timing thing. If you tell a line producer, ‘OK, we need Sarah for two hours on this day to do commentary,’ the line producer is going to tell you, ‘Uh-uh.’ So that was why I didn’t do it during the show. And if I had free time, if for some reason I was getting a day off, I was taking it. I was exhausted,” she explained.
Season seven of BUFFY comes out on DVD on November 16.