The cost is a factor but so is maintaining the radios. Eventually any group that's attracting new members has to decide to keep recommending CB or convince existing users to use FRS. Finding a CB is getting harder without Radio Shacks around or even many truck stops. You can get FRS radios at Walmart or Best Buy instead of eBay or garage sales. Heck, I'd wager that there even more FRS radios on Craigslist than CBs at any given moment, so finding them for next to free isn't any harder than a CB.
Then factor in a CB antenna, mount, installation and tuning. Fewer people now than ever really want to do that. Those that do are already GMRS or ham users by-and-large so CB isn't an important factor for them.
It became obvious that CB was dead in our group to me on a recent run. CB was completely duplicated by ham with people who've been around a while and every new member already had an FRS radio plus an extra or two to hand out to the people with no radios at all. They use them at the park, ski resorts, Disney Land. There's no reason to swim upstream trying to get newcomers to get onboard with CB that they're only going to use in their truck.
It may be a regional thing. CB is heavily used in Mexico, and with so much back and forth between Texas and Mexico well, you basically can't swing a dead cat without hitting one around here as the saying goes...
Yes, Radio Shack is gone, but in the audo dept at just about every Walmart I have been to there is typically a radio or two to chose from, along with at least Wilson mag mount antennae.
And honestly, most of our budget oriented members, young guys typically, are recycling dads / uncles / grandpas old CB radio and mag mount antenna.
And again, budget oriented members tend to cruise the swap meets for parts, and yes, lots of CBs and CB antennae there at nearly free prices.
I can literally go point you now to no fewer than 10 vehicles within 2 blocks of my house with either 102" whips or those coil type antennae...