My guess is that there could be cable clutter on the floor. Or, more likely, they're worried about dust. Computers do get dirtier on the floor and are less likely to be cleaned. Dust inside the box is not good.
I've recently been informed that my computer at work is a "desktop" style computer & may not be placed on the floor, but rather must be placed on my valuable & space-limited desk. Is this true? What do the manufacturers say?