Before you say "oh maybe they have an invisible disability" - I seriously doubt that the bus is full of these twice a day, every day, which is how often I take the bus.
I get on at the terminus so it's empty. Without fail people would rather cram themselves onto the bottom or sometimes even stand than go through the trauma of hauling their asses up to the upper deck.
How bloody lazy are people, it drives me crazy! One girl actually preferred to take the back seat and sit there mouth breathing and facing three of us rather than take 8 steps