The baby's skin is thin and delicate. His skin is only half the thickness of an adult's skin.
So when the baby's skin is exposed to heat and moisture due to the presence of pee and poo in the diaper, it becomes wrinkled and is more prone to damage.
Furthermore, the pee and poo contain bacteria that may irritate the skin, making it susceptible to diaper rash.

The baby's bottom is exposed to heat and moisture which makes the skin prone to rashes. When factors such as pee, poo, bacteria and the disintegration of diaper against the skin are present together, they become skin irritants that are likely to cause diaper rash.