Because they suck? Triodes suck too... why don't they use 'em anymore, right?
I don't know any SIT characteristics offhand, but evidently they simply aren't competitive to MOSFETs and BJTs. So, yeh.
Your concern is misplaced -- linearity is a non sequitur for amplifying devices.
As long as the operating range is continuous and one-to-one, it doesn't really matter. What does matter is how much gain is available in general, what saturation voltage is, and device capacitance or inductance. We can fix everything else by simply using a couple more devices and applying negative feedback to clean it up, and maybe a few more to re-distort it back to whatever other characteristic we want.
Whereas no amount of feedback will gain you back the pitiful efficiency that triodes have, or the high impedance and relatively low bandwidth of vacuum tubes in general. (You can use a triode in positive-grid operation to reclaim some efficiency -- which costs some distortion due to grid rectification loading the driver -- but this takes more effort, and again probably needs feedback to manage the distortion.)
RF amps are the most stringent, as there's no time (that is to say, phase margin) for anything more than local feedback -- usually shunt feedback as a output-to-input resistor in a common-source configuration (often called "neutralization", but it's not just compensating for reactance when it's used in this way). In this case, it does help that conventional devices are much better behaved than they possibly could be -- an exponential transfer function is a far sight better than some lumpy sort of thing you might imagine. So we get the basic calculus advantage of, small signals span less curvature so have less distortion; and a few tricks like push-pull or other phase shift hybrids (for narrowband RF mostly) to cancel out some harmonics/IMD. And if you don't mind spending a lot of time figuring things out, you can predistort the signal to perfectly cancel it out with the final's distortion, which is how particularly high linearity power amps are made, for cell towers etc.
Tim