Oden - simply because if you are asking the AFCS to maintain both a specific airspeed and a specific height it will inevitably result in continual adjustments to collective pitch (unless its flat calm). That means control runs moving, swash plate etc moving far more than would be required just to maintain attitude, although of course the control systems are designed to be able to do that.
Probably the main reason is to reduce the changes in N1 (Ng or whatever you want to call it) that result from changing power demands. Gas turbine engines like to operate at constant speed - every time the N1 increases or decreases speed, that alters the centrifugal (alright, centripetal for the purists) force on the compressor and turbine blades, effectively stretching the blades more or less. Each change in rotational speed is therefore effectively a tiny fatigue cycle. I am sure engines are able to tolerate this, but if there are 2 ways to do something, both equally effective but one of those ways results in less wear and fatigue, even if only slightly, why not go for that method?
HC