No reason in particular, but I don't have a Facebook account so integration with Facebook would have required me to sign up first to test the integration.
I don't mind creating an account on Facebook if required, but I am trying to avoid it if possible.
Oh, I just got downvoted, unsure why. I was thinking that it's a pretty neat tool to integrate with Facebook. As your primary goal is to build your family structure, you may want to share it with your family members (and maybe have them help building the tree).
Also, depending how you want to grow, fb login is typically the most successful one.