well it's just evolution isn't it? Survival of the fittest.

Under the current setup it is beneficial to either build a spinner, rammer or flipper simply because those are the strongest machines in the arena. It's been a long while since a walker came along and I've never heard of a hovercraft, unlikely to win. Crushers and axes, the limiting factor is the weight. There just isn't enough weight in the category to allow construction of machines that will get through the tops of current machines.

Change the environmental conditions and the type of machine that is top dog might well change.