2) Already have a pretty talented young finesse pf in CV.
3) Might be unhappy in Milwaukee and try to force his way out.
4) Hasn't proven he's the next Nowitzki or anything, so far just an unusually athletic 7-footer with a decent jumper, not necessarily any other remarkable attributes like competitive fire or creativity or great defense.
5) Rumor has it Kohl prefers CV (okay, basketball-wise that's not a good reason to trade Yi but it is up to Kohl in the end)
6) Yi is the one trade asset who can make the Bucks much better now, whereas trading one of the veterans would be taking a step backward in hopes of eventually taking two steps forward. One could argue that the Bucks need to be moving forward now in order to appease their owner and fan base.
7) Goes with #6, but Yi could fetch a really good player in a trade, possibly even someone like Iguodala, Horford, or Okafor although a Yi + bad contract for someone like Marion or Gasol is more plausible.
