FPS games kind of suck on a controller, due to a lack of precision. At the very least, you can't compete with anyone using a mouse.
A trackball, on the other hand...
A trackball, on the other hand...
And on top of that, occupy some 'third' control scheme somewhere between "kb/m" and "gamepad" that developers would have to explicitly target, at a time when so few of them even put the time into adding proper kb/m support for their console-first-PC-port-maybe-if-its-cheap-to-farm-out titles.