They are easier to handle (lift) for 1 or 2 person-teams. Some Roboteers sell their HW due to bad backs.
On all other points, I agree with Jonno.
I do think that it is not hard to build an underweight HW, these days. With lighter batteries and a tendency to build denser robots.

If you want more spectators, build more spectacular robots; faster (minimum speed?), with active weapons and sacrificial armour. Minimum ground clearance for the bodywork eliminates being stuck on the arena floor. Minimum ground clearance for the flippers will give others a bit more chance.
Good-looking robots are great, but noisy robots pull in people from the other side of the hall.