The primary one is that I need to flex my web skills regularly in such a way that RapidWeaver didn’t allow. I can tweak WordPress at a very low level whereas RapidWeaver keeps things at a very high, GUI level.
I also wanted to resurrect the blog, but RapidWeaver’s built-in blog tool doesn’t really suit my needs. I had to have RapidWeaver installed and running to post, but with WordPress I can manage the site from anywhere. I didn’t want a separate blog site where the designs didn’t match (or rather, I didn’t want to take the time to tweak the designs to match).
None of the above is to say that I don’t like RapidWeaver; I still use RapidWeaver for another ongoing project. I think I just wanted to add something new to the mix, so WordPress since it is new to me.