Political parties, being more-or-less coalitional, actually need to take positions on a wide number of issues to be able to draw in people who are oriented towards things other than the party’s main issue—that is to say, one might think both parties are bad when it comes to good government issues, but one still probably lines up as a D or R when it comes to entitlement spending, the deficit, foreign policy, etc. A lot is needed to uproot people from where they are.
Or, to put it another way, there just aren’t enough people who care about politics who’d support the “not stupid or evil party” just because it’s not stupid or evil.
And, many observers suggest Roosevelt would have won the Republican nomination—and almost certainly the presidency—had the 1912 convention not been stacked with Taft patronage appointees from “rotten borough” delegations from the South. I don’t know that there’s a specific lesson for John McCain in there, but the route to power is much easier if you can take over a major party than starting your own… ask the Christian Coalition or the Deaniac crowd, who now effectively control the two major parties, if you don’t believe me.