Why Not Bonds?

Ray Ratto on ESPN.com asks why Congress didn't subpoena Barry Bonds in their steroid investigation. He suggests that the reason Bonds was not included in the group (comprised of Canseco, McGwire, Sosa, Schilling, Giambi) is that Bonds wouldn't offer much. That doesn't seem to be a good reason not to ask! Also, why wasn't Sheffield asked to the hearing as well?