Once Kris Benson got signed it became impossible for the A's to even attempt to keep Hudson. Benson is not even half as good as Hudson and got 3YR/$22M. Hudson has like the 3rd best winning percentage of all time despite having no run support. Once shitty pitchers like Benson start getting signed to ridiculous contracts, the value of pitchers who are actually good like Hudson skyrockets. They traded Hudson cuz they'd have to let him walk and would rather have someone who can contribute now or very soon rather than a draft pick. Same with Mulder, I think the A's know he's not going to be good anytime soon. Zito stayed cuz if I remember correctly, he's always pitched best in the most important games, he's a Cy Young winner, and he's never been injured. His being a fan favorite was probably a big reason too.