Could be because Graham plans to move for reconsideration. Any voter on the winning side can do that.
I’m not saying that was his motive, just tossing out a possibility. Senate protocol usually has the party leader make that motion, and most often the protocol is vote is rendered final bay making and tabling motion to reconsider on unanimous consent.
Senate is likely negotiating the number and form of witnesses to be allowed.
Democrats and the Graham wing of the GOP must have decided the impeachment theater is good for them.
Bump