#1) WPLK's daytime signal in Duval County/Jacksonville is marginal. AM signal propogation all through Florida is horrible because of the sandy soil. And WPLK's night signal is completely inaudible in the city. The first rule of ratings-getting is "They gotta be able to hear it."
#2) Sebring's population is much older than Jacksonville's, so "Music of Your Death" is pop-pop-popular stuff down there in Senior Villas, but not so much in Jax.
#3) Jacksonville has 5 times as many radio stations as Sebring, so EVERY station has to fight tooth & nail for ratings, comparatively.