A lot of people aren't willing to move to it because the challenge from a lot of things in TFC is either lessened or completely removed in FF. I personally wouldn't mind giving it a shot, but a lot of the top players say they cbf with FF.
A few examples:
You can hold space to bunnyhop in FF
Sliding up ramps requires you to press duck at the right time in TFC - in FF you can do it without pressing anything
When you are hit by a concussion grenade, it screws up your screen and your aim is not determined by your crosshair - in Fortress Forever, each time you shoot while conced, your true aim shows up on the screen
That's just some of the stuff that becomes p much trivialised in FF. As I said, myself and a few others of the AU TFC community wouldn't mind giving it a go, but the problem being that it would divide the community to the point of a decent amount quitting playing.
bloody stubborn lot