>>213717177Because voldemort had already dropped the mask by that point.
He just wanted to get on the grounds so he could put the horcrux diadem in the room of requirements.
He had already made the huffelpuff cup a horcrux, and slytherine's locket.
He really wanted to complete the set but the sword was the only item of gryffindor's still kicking around and dumbledore kept it in his office.