Cost is the most likely reason. That being said, drums work just fine. Especially since we are talking a mid sized pickup.
I really do get baffled at why people get hung up on rear drums. It's not a big deal, if it had discs, you probably would see a dramatic change in braking.
Front brakes are a different story.
I have read where Discs were added from a 4 runner with less then stellar results
