I can think of a few reasons:
Not enough room for it without major surgery
the drivetrain would have to match, and there isn't enough room/length for it in a 5
the torque would crack/twist the frame like a pretzel
not to say it can't be done, it has been done. Just not practical for a street/trail truck, just a drag or mud racer. I actually thought about it with the rock crawler chassis I have, until I really thought about it.