I Just don't like Kucinich at all, he maybe against the war and for the impeachment of that monster cheney, but he's a socialist who supports the UN and open borders.
Kucinich isn't polling high enough for it to be worth Paul's time. Right now Paul is at 8% in early states and 6% nationally. Plus, he's still running in the primary, so he needs to debate a Republican. He needs to debate McCain or Thompson, not an also-ran like Kucinich.