If you really want to know what drove someone to sign up for your page you need to ask them. The best way to get an answer is to reach out personally outside of any automated mail.

You want to thank them again and ask them what drove them to sign up to your page. You'll learn what features and benefits you should be pushing even more this way. You'll also learn about what could be cut from your page. Ideally your page has only the content required on it to get people to convert.