Bathroom floors (and bathtub surfaces) are slippery. Bathroom fixtures don't make cushions if someone falls on or against them. People with disabilities or injuries don't have much choice but try to find a way to shower or use the bathroom, so that doesn't help. Children can be fast-moving and careless. People who are sick or drunk probably spend some extra time in the bathroom. I suppose there's the chance, too, that if a person starts to develop serious symptoms his first instinct may be to head for the bathroom, rather than call 911 (if he doesn't know what the symptoms are). Then there's the chance of water and electrical plugs being "a problem". The water in tubs or toilets can lead to drowning (of adults or children). Probably, too, no matter what other rooms people use or don't use, everyone has to use bathrooms (so they must get more use).
These are my guesses.