Among other things: One, wepay isn't international. Two, you have to send users off your site to use it. Three, you can't plug in your own credit card processing if you use wepay (you have to use theirs).
Additionally, their api doesn't provide the tools for the collaboration side of things (messaging, invites, sharing, comments/nested comments, reward tiers, tilting mechanism, etc). Think of this api as a layer above credit card processing (so you get to choose whichever processor you like or already use, thus not disrupting your current flow of funds and you get to keep the branding/UI/UX, the users, the user data, and the credit card processing data, etc - all important things if you're, say, hotels.com).