They didn’t.
These are the station ID letters assigned by the FCC to regulate their transmitters. They are legally required to say or display them pretty often, so the marketing guys see this as a branding opportunity.
K means west of the Mississippi river and W means east of it, the two regions the FCC established when they started the station identification process. I think you can request letters that are not in use, but otherwise, you just get the next one off the list.
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