I'd assume it would just be because of the general rule is that petitions are only allowed on threads on the Petitions and Activism board?
It's irksome, especially as petitions on that board tend to gather tumbleweed, I wish that rule could be reviewed but I believe it's to stop people spamming threads with petitions. This is mentioned in the guidelines, though it's not very clear that all petitions are liable to be subject to this rather than just the spam ones.
www.mumsnet.com/info/netiquette
www.mumsnet.com/Talk/petitions_noticeboard