While over here in the UK we moslty have bi-metalic switch start fittings.
I think this is because the pre-heat ballasts that keep the fillaments powered continiously are autoto transformers that also boost the voltage to about 200V; 120V isn't enough to power a 6' tube. We have 230V which is more than enough to power a tube so there's no need for a transfromer which would add to the cost of the fitting.
I bet the switch start fittings you have seen are for the shorter tubes.