Because he's Typhoid Mary for the horrible disease called "racism". The US suffered tremendously from this disease for many years and though it has died down somewhat, it is still virulent and powerful and can flare up when a powerful strain is injected into society. While it is true that Santorum will probably never hold an important office again, he can still be a firebrand and a carrier for racism, homophobia, mysogeny and other sorts of diseases of hatred that plague civilization. We can only hope that exposure to Rick Santorum, like some vaccines work, will make our people's resistance to such ugly infections even stronger. But that's not a sure thing. He's certainly built up a colony of hate-infected people around him, so there must still be some people susceptible to the memetic germs he carries.