Why did Dennis Kucinich join with pro-war Republicans to try to rush the Cheney impeachment motion to a vote?

Because the Republicans adopted the often used Democratic strategy of letting Dennis make an a$$ of himself. The Republicans bet there was a greater chance of Dennis failing and potentially making a fool of himself than succeeding so they waived their right to "debate." The Democrats discreetly forwarded the complaint to the judiciary committee where it will sit and rot. It's to no one's advantage to impeach right now.