There are a few reasons.
The first is that it falls into a realm that's perilously close to spam. It's unsolicited email with a purpose. At least the purpose isn't commercial, and I appreciate that. Still, it's not the kind of thing I would send out. When I do send out mail asking people to do something for a cause, I point them to a web-page.
Secondly, another problem with email petitions in general is that the person receiving them will have a lot of duplicate "signatures" to sort out. Everyone you sent the petition to who forwards it along will be creating a petition with all the signatures before yours duplicated.
To be effective there must be a way to uniquely count the people who've signed. Email petitions make that particularly hard to do, given the circuitous trails an email chain-letter can take. They're not particularly effective in general.
To quote from http://www.scn.org/~alf1701/activism.html (There are additional links at the end of that page, as well.):
Only three times did anyone speak up about the limited effectiveness of chain letters in such situations, or pass on the report that the well-intentioned originator of this letter had been swamped with email far beyond her expectations, and her university had canceled her email account.
Thirdly, I do not forward chain letters, whether in paper or email. If bad juju rains down upon me for this, so be it. The CIAC has a good summary of the issues at their Internet Chain Letters page.
Fourth, (in response to the Taliban Women email) this particular email petition has been circulating since about the start of 1999. MacKiDo Spam Alert and The Mining Company's Petition for Afghan Women's Rights were posted to deal with it. Another page explaining why this particular petition is useless is at Wooden Spoons. Madeline Albright made a speech in CA saying the US won't abandon the Afghan Women. The article is at http://www.timesofindia.com/today/07worl5.htm. That's a bit of good news. Sending her a thank you letter would encourage that sort of thing. State Department Feedback or Mail the Secretary of State should reach her.
Lastly, by replying to you at length like this, perhaps I'll push you (or someone who sent you the email) to set up a web-page and collect signatures in a more effective way.
A web page which contains a lot of good information is at http://www.netaction.org/training/v-training.html. Specifically, it tells you how to do a web-based petition and why it's so much more effective than email. It also tells you how to make it most effective. For example, putting on a "sign by" date, so the petition doesn't continue to circulate forever, like the "send mail to the dying kid in England" (Craig Shergold), who got better 10 years ago, but he still gets mail.
Hope this helps,