QD rings as a scope makes cleaning a muzzleloader a much more difficult process - plus some of the solvents people use on their rifles can be a real hazard to a scope's optics and finish. So two reasons - easier cleaning and protection of the scope.
ps: the Warne base offers about the most flexibilty so far as scope positioning - the only one I can think of that offers more is theone you will see at
http://eabco.com/cssmts.html (I think that one may be aluminum vs the steel Warne).