The vick thing was a little different and easier to prove. To prove the perjury charge they will need to prove beyond a reasonable doubt that he did take performance enhancing substances (roids) and prove beyond a reasonable doubt he lied about it. It's hard to prove those and they needed to get all their ducks in a row. I think the last duck might have been his former trainer who sat in jail for something like two years on a contempt of court charge for refusing to testify. Oddly, he got out only a day or two before the indictment was handed down.