CallStack naming

Hi devs, I'm sure there's an easy answer to this, but I'm wondering: why is the CallStack feature implemented with implicit parameters instead of just a magical constraint? Whenever I use this feature, I don't want to have to enable -XImplicitParams and then make sure I get the name right. What would be wrong with, e.g.,

undefined :: AppendsCallStack => a

Seems simpler. Is it problems with a nullary class? Thanks, Richard