Because Lupul, at the time, was considered more of a throw in. Gardiner was the price to pay for taking on Lupul's contract. Which at the time had 9.5m+ left on it I think?
Without going to find articles on it, I'm pretty sure even Burke said that the goal was to acquire Gardiner in the trade. He was the 'prized piece'. I think they liked Lupul and liked the potential for Lupul to pot 20 in a secondary role, but I don't think they expected the type of impact he has had. I also don't think they expected Gardiner to step up like he has. I remember them saying how he might have needed half a season in the AHL first.
I don't blame Anaheim for moving him. He wasn't exactly healthy since they acquired him from Philly, so I can imagine how that would be frustrating for a playoff team. They watched him suffer through injury, and the door opened for them to acquire a top-four defender in Beauchemin for a playoff push (while shedding cap space in the process). They saw an opening to move the contract and took it. Moving Gardiner was from a position of strength for them.
Good news for the Leafs that he seems to be motivated, and that he found chemistry with Kessel. It's these types of moves (Phaneuf, Lombardi, Lupul) where the Leafs have been able to flex their financial muscle + acquire extra assets for helping another team out of their cap issues. It's the type of deals Burke (trading cap space) has been wanting in the NHL for years - just a bit of a style to it.
While you are correct in that Gardiner was the 1st target, I remember Burke as being adament about Lupul being able to do everything he is doing. He went on a speech about how much he has always loved him and that the only thing that got him to trade him before was Pronger.
I do not believe that Burke looked at Lupul as the "throw-in" that the Ducks or the Leaf fans thought he was when the trade happened.
Not the exact quote I was thinking of but it illustrates my point
Thanks Leaf Rocket. No idea how Burke managed to swing that ridiculous condition ("If he does well for you, we'll give you more stuff!"), but I really do love that deal. One of my favourites in recent years.
EDIT: Didn't realize it was for the 2012/13 season. Not that it makes a big difference, but Lupul was obviously looked at as a piece that would either get re-injured or would be ineffective, which would make it tricky for Burke to move him, especially with the price-tag. So I guess if Burke is "stuck" with him, we get a better pick. Still very faulty logic, but I won't complain