I'm reading a book about women in the American abolitionist movement. There are a lot of similarities between that movement and today's animal rights movement (such as it is...but that's another post).
One of the differences I see immediately is the role of the churches and the religious community. Quakers, for example, were the early abolitionists in the US. The drive to emancipate slaves was grounded on religious and moral grounds. Slavery was a "sin" and to work to end slavery was "God's work." There was tremendous moral energy, organization, networking, and credibility that came from the involvement of religious organizations and individuals.
Now, I'm an atheist, but I'm practical enough to realize that religious groups offer a certain weight and a lot of passion to issues they choose. Just look at the pro-life movement. Where is that religious outrage over the treatment of animals?
It's not there and it is one crucial element the animal rights movement is missing.