In short, because Joan and her management company are a bunch of selfish bastards.
The full story can be found in link
مضمون and interview with the documentary's director/former باس, گھنگھور player Victory Tischer-Blue. I've kinda summed it up, if آپ don't feel like reading all of that:
Joan & Co. did everything in their power to stop Edgeplay's release - the version you've seen is very different than the original. Through several lawsuits, Vicki lost the rights to use any Runaways songs that Joan had a part in creating - which is why there's so much of Lita's and Suzi's music.
As for the reason why Joan isn't in Edgeplay... according to link
article/interview (also linked above), Joan and her scumbag manager/advisors thought that Joan deserved مزید screentime and that "far too much importance was placed on Jackie’s and Vicki’s involvement in the band…" The record company that holds the rights to The Runaways music, in fear of a lawsuit from Joan & Blackheart, decided not to license any Runaways songs for Edgeplay that Jett had anything to do with.
Joan has also gone out of her way to try and sabotage Lita's career (see the paragraph on the last page of the مضمون about the song writer/music video director). I read in another مضمون a while back, which I can't locate at the moment, that through yet another lawsuit, she had the subtitle of Edgeplay changed from the original "The Runaways Story" to the one that appears on the DVD, "A Film about the Runaways." Guess it can't be factual if one out of six member refuses to participate.
In MY OPINION, which has nothing to do with this article, Joan may have already had plans for her shitty movie and didn't want any competition. Or, y'know, anyone to know how full of bullshit her failure of a movie was. I think the plans for the movie, combined with the fact that she wanted the documentary to be مزید about her than the other members, may have been part of her motive for trying to stop Edgeplay from releasing.
In conclusion, Joan Jett is a selfish bitch. The end.