Why is the sup.actress category always the weakest one?

It's because it's the dumping ground for coattail nominations, category frauds and veteran prizes. It's not that there aren't great performances by actresses in supporting roles, but a lot of the time, the ones in Oscar contenders tend to be underwhelming because they're usually suffering/supportive spouses, suffering/supportive mothers, or children who should be lead but the Academy can't reconcile that.