Band 20 LTE/800MHz allows the carriers to vastly increase their coverage area for a lot less money.

Customers can only access that spectrum if they have a VoLTE compatible (including emergency call) device.

The carriers have legally binding landmass and population coverage goals as part of the spectrum licensing terms.

The carriers can also market the fact that better rural coverage is available on newer devices.

Therefore the carriers have only been selling fully VoLTE compatible devices for many years and pushing as many VoLTE-enabling software updates out as possible.