Because it wasn't a priority. Plastic is more expensive than concrete. It's only now that "look how green we are" is able to tip the balance to where it's worth implementing.
If someone really wanted they could come up with a more complicated counterbalance solution and probably eliminate the dead weight entirely.
I really want to see how they came up with their numbers. I suspect that they're overly optimistic. +/-100lb isn't going to matter for fuel consumption in the last mile and the CO2 cost of picking up and replacing one that failed under warranty would likely negate many hundreds of machines worth of reduced fuel consumption before the last mile.
If someone really wanted they could come up with a more complicated counterbalance solution and probably eliminate the dead weight entirely.
I really want to see how they came up with their numbers. I suspect that they're overly optimistic. +/-100lb isn't going to matter for fuel consumption in the last mile and the CO2 cost of picking up and replacing one that failed under warranty would likely negate many hundreds of machines worth of reduced fuel consumption before the last mile.