I'm not sure how accurate my knowledge is on trucks that race but...
Truck have more than 6 gears I think and they run a H-pattern gearbox, and with most of todays sim racing equipment the most gears you will find in a H-Shifter is 8, so bringing trucks into LFS, a simulator, would be kind of unrealistic. Yes, you could use a sequential gearbox but it would hamper the realistic side to LFS.
I know many pro sim racers opt for flappy paddles in GTR cars and other cars that are meant to have stick shifts but I for one prefer to keep it as realistic as possible and therefore use the appropriate shifting mechanism as according to the vehicle I am driving.
Although we do have people racing round using a keyboard or mouse, which I acknowledge in real life, would be pretty difficult.