Why do we go down slower down a slope with less air pumped in a bicycle?

Well, the bearings doesn't care much if it's a 120 lbs rider or a 180 lbs rider - their drag is very much the same. The tires can easily be inflated a bit more for the heavier rider, making rolling resistance pretty much the same too. What's left is air drag, and the clue here is that a rider that's 50% heavier won't have a 50% bigger frontal area. So the lighter rider will only have a tad less wind resistance but a much lesser weight dragging him downhill.