I would think that the reason why the opted for the `all moving horizontal stabaliser` is because it has more authority than the conventional elivator type system, and on a ac of this size it would be more practice than having a elivator on a aerofil. If you look at PA34 TB9/10/20/200/21 there all the same, mainly because the speeds involved arnt that high, therefore at slower speed greater deflection is require and possible with this system. and its probably cheaper to produce also. I hope this helped