Why Now?

I'm not sure how I feel about Rep. Mark Souder (R-IN) resigning after admitting an affair today. The fact that Souder is an abstinence-only crusader adds some irony, the fact that he shot a video promoting abstinence with his mistress adds yet more. But while I am pleased that for once a Republican actually has to leave politics due to sexual shenanigans (a Democratic affair, Bill Clinton excepted, seems to be death-on-sight), how is it that Souder has to resign while Senator David Vitter (R-LA) is still around?