I guess Cox wanted to hold onto WFOX for possible radio use in the future and the opportunity to place them on a Cox owned Fox affiliate made it worth keeping. I expected that Cox wanted more than Fox was willing to pay or Fox was happy with WNYW in New York. Cox use to own KFOX in El Paso the Fox affiliate there before selling it to Sinclair.
For the record, WFOX-TV is currently the Fox affiliate in Jacksonville FL (RF 32, virtual 30).