I guess it is to differentiate between these and the preexisting walkers who will NOT be allowed as commanders.
It sets an interesting precedent too, could be just as easily slapped on an enchantment or artifact or somesuch.
I think Walkers can work just fine as commanders, assuming they are balanced with the idea of doing so in mind, which some are not. Sorin 1.0 for example, I don't particularly want to play games against an immediate 30-life point swing that I cant have a chance to disrupt beforehand. Sarkhan Vol? Now THAT GUY would be a cool commander... But once you start making exceptions for a few, it gets weird.