Why Carpool sticker should be on bumpers not on the windows?

I am wondering why the carpool sticker should be on the bumber and right hand side. There are a lot more better place to stick it. I do not have carpool sticker but I saw them a lot on the hybrid cars and I am JUST Curious.

Putting it on the window would make it harder to see out of the window. The right side of the bumper was selected because it is visible to vehicles that are in the lane to your right (the car pool lane is usually the far left lane) and does not affect your ability to see.