You'd trade a 22 year old kid with a huge potential that he has already tapped into, and an amazing low post game for a guy his age (along with the 5th pick, possibly Gerald Green (god knows how good he can be), and a filler or two?) for an aging 32 year old guy with no experience winning?
Yeah, Garnett is a better player at the moment, but in 1-2 years Jefferson will surpass him, no doubt in my mind.
Garnett isn't a winner, and it'd be 10 times easier to teach Jefferson to become a winner than to teach Garnett to become a winner.
EDIT: Al is also going to be a top ten post player this year, no doubt in my mind. He'll go for 20/11 this year.