Sandy,
Actually, the AM CB band currently has 10 khz channel spacing, with a couple of exceptions, and 2M FM, in So.CA at least is running 15 khz spacings for repeaters and simplex channels. We here have even "pretty much channelized the simplex portions as well. TASMA, the spectrum coordinator publishes "Recognized" 15 KHz spaced simplex channels for the band plan. Many areas of the U.S. use 20 KHZ channel spacings. Twice as wide as current AM CB. The SO cal plan for 10 meters, which would have very similar propagation to CB also uses 20 kHz channel spacing for FM.
Also switching to FM would obsolete a lot of existing CB equipment, and given the "screw the rules" attitudes of many CB'ers, they would probably continue to run their existing AM radios. That would make co-existance of the two modes with two different channel spacings on the same piece of spectrum pretty difficult. Remember, also, even in the ham world, with lots more spectrum assigned, it is illegal to run FM below 29.5 MHZ because of the greater bandwidth used by FM. SSB, AM and other narrower modes prevail below this.
Given all the above. it is highly unlikely, in this age of scarce radio spectrum, a wide mode such as FM would be considered by the FCC.