Could it be because of what he's willing to say?
If there is anything that has pushed me from the "conservative" position on immigration, it's been a combination of not just the slimy underside of this, but the way that those who question it have been targeted by the restrictionists.
Remember how Michael Crichton discussed eugenics in the appendix to State of Fear? It seems that this slimy aganda may have found a home on the right in the form of the anti-immigration movement. Is that why Tancredo has gone after him, to shoot a conservative willing to expose this slimy movement?