Because they don't have the attribute 'light'. I think only two light weapons can be equipped for dual wielding, for instance daggers and short swords. Rapier don't count in it, which is a pity, cause I was looking forward to dual wield rapier + dagger.
At level 4, you can get another two weapon feat that will allow you to use non-Light weapons - so you can do the rapier + dagger combo
