Because Ford's name is synonomous with sleazy Memphis politics. He may have a clean nose, but his daddy, a racist slimeball, is pulling the strings. The rest of the state would never elect him to the Senate (assuming the 'Rats would even ALLOW him to have the nomination (they told him "no" in '02) - and the nat'l 'Rats got their "token" in IL with Osama bin Barack, and the White 'Rat power establishment doesn't want any more of "those Black people" in the Senate), and let's not forget that despite his "moderate" rhetoric, he IS leftist.
Well, he's not THAT left. The American Conservative Union Gives him a lifetime rating of 20, with a 40 in 2003. Compare that with ultra-lib Pelosi's 3 and moderates like Sen. Bayh with 22 and Lieberman with 19. So Ford is squarely in the "Blue Dog" camp.