The battlebots ones are heavier aren't they? You won't see a robot of a different weight class featured.
Actually, I just checked this, and it's not much different 250lbs (~113kg) vs 110kg. (Battlebots vs RobotWars rules)
Actually looking at those rules, it's the details that matter more. E.g. Battlebots robots can use air or O2 but NOT CO2, Robot Wars robots can use air or CO2, and the pressure limits are several times higher for Battlebots.