up a significant percentage of the floor space. Plus standing on the top deck
of a double decker is unpleasent at the best of times with the nausea inducing
swaying, heat, people squeezing past to and fro at every stop. Which is
why most standees stick to the lower deck I should imagine.

It's also because you're not permitted to stand on the upper deck.
Though some (esp night) bus drivers don't enforce it, I've waited for
several minutes before now while the driver sits at a stop pressing
the "Please do not stand on the upper deck or stairs" button until the
passengers take the hint