Because you have a CrewCab, which means it's a long way from the back to the tranny to the front of the pumpkin. Too far for a one-piece driveshaft to work without vibration problems.
SuperDuty trucks with even longer wheelbases than yours have a 3-piece driveshaft. And some PSD pickups with shorter wheelbases - such as regular cabs - have a one-piece driveshaft.
I'm not any sort of engineer, so I don't know the numbers. But basically the longer the piece of driveshaft the greater the tendency to flex and bend and thus vibrate.