Humans have as much hair as other primates - it's just much shorter, and conspicuous only in some parts of the body, in a distribution that varies with age and gender. Nobody knows why.
Why don't human have fur?
There are many theories. Some have suggested it's a relic of our ancestors passing through a relatively aquatic age in which they routinely swam.
Another view is that hairlessness a consequence of sexual selection, and connected to the fact that we are highly social creatures that walk on two legs. Could this explain why human breasts prominent at all times, and not - as in other primates - only during lactation or restricted periods of sexual receptivity?