My guess as the answer to why haven't WINS, WCBS and KYW moved to FM? Well, if they're making money on AM and they can put a different format on FM that will make more money, that's preferable, financially. Once they put the all-news station on FM they have two options for the AM: a simulcast or find a new format. The second choice appears to be nearly impossible today.
I think they put all-news on FMs in San Francisco and Chicago for competitive reasons. FM News launched in Chicago, and CBS really thought they had something to be concerned about. As time told the story that FM News wasn't a threat to the CBS all-newsers, they had no reason to put the any of their other all-news stations on FM.
For whatever reason, the 12+ ratings for the all-newsers in NYC and Philly on AM continue to hold up, with some seasonal ups and downs. CBS seems committed to this format ... look at how they're hanging on to WNEW-FM in Washington DC even though it gets clobbered by WTOP. If the listening to their all news stations in the markets where they're on AM only started to drop off, you might see the stations move to FM.
Also: CBS is limited in its number of stations in those markets not only by the regular radio ownership limits, but they have lower caps because they also own TV stations ... in Philly they own two TV stations.