My guess is to avoid asking every user to change their password. From a UX perspective, it's not a nice thing to ask. This has nothing to do with security, where I agree that having one system is certainly better.
Nobody would have to change their password, the suggested change would only affect Kickstarter's internals. You'd still have two different systems in place like in the solution that was actually implemented, but both of them would contain a 'brcypt'.
The point of carbocation's technique is to avoid requiring users to reset their passwords, but to protect every user with bcrypt. The existing SHA password hashes could be used as the input to bcrypt.