I like to know the medical reasons.

Water is not generally recommended for accident victims, especially if they are unconscious or have suffered a head injury. Here are some reasons why:
Choking
If the victim is unconscious or unable to swallow properly, water can cause them to choke. An unconscious person can also choke on their own vomit.
Respiratory issues
If the victim has a severe spinal cord trauma, liquid can be inhaled into the lungs, leading to pneumonia or other respiratory issues.
Blood pressure
Giving water to bleeding accident victims can increase their blood pressure, causing wounds to open.
Blood loss
Giving water to accident victims is equivalent to mopping away all the remaining blood that would have sustained them for a while.
Medical treatments
Water can interfere with certain medical treatments or procedures that may be necessary after an accident.
Swallowing
If liquids are swallowed, they may enter into any open wounds, which increases risk for infection and further tissue damage.