I mean, he doesn't. He didn't seem to be the right fit for that team's style of play and their top 6. He's a straight away speed player, north-south type of game with his wheels and he's far better at handling the puck, than the Leafs fans are wanting to give him credit for, the usual "we traded away a player, let's crap all over him" stigma.
It's like we Pens fans, the original drafting team, didn't dissect his game the first 2yrs he was a Pens prospect and knowing what his strengths were and then saw him work on his flaws on the Marlies (2-way game) and suddenly show he's capable of much more. He's a lot closer to Hagelin/Rust's style of game, with better hands and shows potential to produce more consistently (closer to Rust but better).
It's funny, Pens lost this trade because people assume Kapanen is just a 3rd line player.
Whereas, when he does play in the top 6 and produce, it'll then shift focus on how the Pens gave up a great prospect (whomever the leafs draft at 15th) for him. There's no win scenario according to leafs fans for the Pens.
The fun part for me is that the Pens got back Kapanen and gave up an ok bottom 6 prospect, a solid 4th line player and a 15th pick for Kapanen and they're still paying Kessel.