The gases used such as SF6 are much better at quenching the arcs produced when switching. Draining it and applying a vacuum would almost certainly cause the gear to fail especially if subjected to high fault switching.
It depends on the details but SF6 provides different arc characteristics than vacuum (a metal vapor arc). There are some very large vacuum arc interrupters but the devil is in the details of the design. In some applications, they actually blow a puff of SF6 gas into the arc.
below 33 kv vacuum cb's are preferred
above 33 kv sf6 breakers are preferred
I think required vacuum is not obtainable for above 33 KV
