The reason for FOX and its growth in affiliates is largely due to Clear Channel's wholesale "partnering" that put FOX on the map with a couple of hundred affiliates essentially overnight from the beginning a couple of years ago. Many of those affiliates were ABC and CBS stations that were dumped and not recovered by other stations in many markets, while other stations went to lengths to pick up the ABC & CBS affiliations ... but not in all markets.
ABC was dumped by a lot of conservative talkers by Clear Channel and others because of a perceived "liberal bias" that FOX was set to fill and reverse. As a result, ABC lost affiliates of its Information Network. With the same reason, especially on top of the Dan Rather fiasco, CBS was dumped as well on many conservative news/talk or talk stations.
Additionally, stations have dumped network affiliations with CNN for much the same reasons, opting for none-at-all in many instances, while Clear Channel has cleared out local newsrooms in other markets of late.
Many stations are opting for cut-away uses of on-the-hour networks now ... getting the first minute or two and the net news, the net spot then dumping out -- recapturing local inventory -- while also cutting back on net feature programming.
Westwood One has become protective of its affiliate list of late because the number of defections and changes ... as well as the number of both CBS and CNN affiliates has been in flux since FOX debuted. (Westwood One also represents NBC News Radio ... a one minute headline service, as well. ABC took the biggest brunt of defections, being on some of the biggest conservative talk stations and has also been pretty protective of its affiliate list of late.)