Steve Benen continues to complain about John McCain dominating the Sunday shows; I continue to complain about his complaints.
My main comment is that, basically, there are going to be Republicans on the Sunday shows, and it's not really up to liberals which ones get on. If Harold Ford is asked to represent liberals, then liberals should (assuming they don't feel represented by him) make a fuss. If John McCain is asked to represented Republicans, then it's really up to Republicans to complain if they're unhappy. At the same time...who cares? Sunday shows are watched by a relatively small group of unusually highly-informed political professionals and political junkies, and those shows neither set the nation's agenda nor have any chance of swaying undecided voters.
All that said, Benen also says: "If there's a good explanation for bookers' obsession with the failed presidential candidate, I can't think of it." So, while I don't think it matters much, I'll suggest some reasons.
Number one by far: regulars are regulars, whether it's who shows up as guests on the Sunday shows or which academics get quoted frequently in news articles, because...they're willing to do it. Odds are that quite a few of what have been 41 GOP Senators just had no interest in spending their time appearing on Meet the Press, since few of their constituents -- and practically no swing voters -- watch that show. Others may not be willing to talk about subjects outside a narrow area of expertise.
McCain, obviously, is willing to claim expertise on every foreign policy/national security topic, and most domestic topics, whether or not he actually knows what he's talking about. And he evidently likes doing these shows. That accounts for most of it.
A second issue is that John McCain presumably is not running for president. The networks try to avoid favoring any particular presidential candidate, mainly because it will spark complaints from the other candidates (I'd guess that McCain's dominance of the Sunday shows tailed off in 2007, during the primary campaign).
Third, it's certainly possible that McCain gets higher ratings than would a generic GOP Senator; as a former presidential nominee (and after 2000 as a former serious candidate for the nomination), he at least is a lot better known than pretty much any other potential guest.
Similarly, the Sunday shows also compete on prestige, and a former presidential nominee probably is, as they say, a bigger "get" than a no-name committee ranking Member. At least in the next Congress the shows can turn to House committee chairs, but that wasn't available on the Republican side in 2009-2010.
And, again, the biggest points are probably the first one here -- that he says yes to bookers -- and the absence of a negative, that conservatives don't complain about him being on.