Technology wise the featherweights are the main driving force. The power of the current generation of featherweights coming out is insane, something that most heavyweights could only dream about during the first 5 or so series of robot wars. The reason is simple, cost. Its far easier to build a crazy powerful featherweight and have somewhere to run it than do the same with a heavyweight.

There will always be a place for heavyweights. As Jonno rightly said, you need the size of them to entertain a large crowd but I sometimes feel that there isn't the same driving force to keep pushing boundaries as there is with the feathers.