I just had a thought about this. I don't think this was given as a reason but it would make sense if it was because it had more processing power as they were only powering the one cyberman at that time. It would be a good way of being able to make 1 cyberman almost as much of a threat as 5 million anyway.
I think my major problem with the Cybers is they still don't seem to DO anything. They just sort of wander around and stand to attention a lot. And given that first one was so fast, why did the others take so long getting into the castle?