Quite simply the answer is ratings and advertising revenue.
Lakeland - Winter Haven became its own market to escape being in the shadows of Tampa Bay and Orlando. It's a Top 100 Florida market, and despite how well any of the Tampa or Orlando stations perform in Lakeland - Winter Haven, local WPCV always performs well as the number one rated station, pulling consisient double digits. If Lakeland - Winter Haven were a part of the Tampa Bay market, or even a supplemental market, that would mean WPCV and all market stations would be rated among the Tampa Bay stations resulting in WPCV not rating anywhere near where it is rated now.
Translation - a combined Lakeland - Winter Haven and Tampa Bay market would mean a revenue loss for the Lakeland - Winter Haven stations, namely WPCV. Each Lakeland and Winter Haven station would be rated in Tampa where the Lakeland - Winter Haven station coverage is inferior. Some of these stations, namely the AM signals, do not have a signal at all over Tampa Bay; Hence these stations would not even make a showing in the ratings. Right now Lakeland's Adult Standards WONN-AM 1230 and ESPN Sports WLKF-AM 1430 garners some ratings points.
The end result is Lakeland - Winter Haven would be rated in combination with Tampa - Saint Petersburg and not rated seperately in Lakeland - Winter Haven as each one is rated now resulting in much fewer agency [and to some degree fewer local retail] advertising dollars for the local LWH stations.