Because that’s his job. Well OK his job was to give Selig a reason to kick Bonds out of the league before he broke the record, but having blown that deadline Mitchell’s job is to come out with the whole report and name lots of names. Comfortably in the off season so the whole thing will be old boring news by the time pitchers and catchers report. This is basically finding a way to officially “end” the steroid era in baseball, of course it won’t really end it but at Selig will be able to declare the facts are in and it’s time for MLB to move on.
It could have the opposite effect by giving cover for Bonds to enter the HOF.