Because it's supposed to work with either bootloader, though the second core should just do nothing if you're running the old one.
Remember: Rockbox is not released yet. If you're using out of date parts of it, it's mostly your responsibility to know that updates have been released.
When the release version comes out, if ever, it'll include a release version bootloader, but until then things will be constantly changing, and you can easily see in the changelog that updates have happened to any given thing.