I actually just read an article related to this very question. In the mid-1800s America was in a time of possibility, independence had been won and black Americans were on the verge of getting citizenship.

Thus women felt it was an appropriate time to ask for more, and to be included in this newly shaped country. Of course, the declaration also was a response to their invisibility, their rights weren't being prioritized. Though they thought they were asking for "a lot", I think they envisioned the battle being easier than it was.

I hope that helps, and for more read the last issue of Women's Review of Books where there was a great article by Vivian Gornick.

-- Amy