different compounds help aswell...

slicks work on the basis that as much of the tyre is on the road as it can be... where as road tyres have to work in all conditions, as well as a much longer time... thats why they have tread patterns and are usually a harder compound, to remove water (las was said above) and how would you feel if once a week you had to change your tyres?

thats what i learnt in my a-levels anyhoo...