If passengers are not paying more for a seat with radio then is is a cost liability to the company.
The cost to maintain it and to pay XM for the service, even the fuel to haul the weight of the equiptment is a cost to the company.
I recall back when we removed all the skyphones from the planes because no one used them and there was a cost to maintain the system.
Then years later we removed all the old wiring to save weight and fuel.
Every cost must be considered, it is the Southwest way.
we are now adding WIFI but are charging for the service which is a revenue positive not a loss like radio that no one would pay extra for.