You don't know why? The consensus is he's a defensive liability, tied to probably more money AAV that he deserves, for 6 more years. I think it was just before the allstar break that he had been on the ice for the second most goals against in the league, not sure where that stands now given he hasn't been playing.
If Dion's AAV was 5.5-6 he would probably look a lot more apealing to teams, hence why so many people think the Leafs will need to retain 1-2m per year to see anything of value. If he was locked up for only 2-4 more years he would be extremely tradeable, but that's not the case. It's the combination of the money and term that I think makes trading him difficult if you're looking to make a hockey trade and not just dump salary or exchange bad contracts.
If the Leafs simply wanted out of Dion's contract they could have moved him last week. Look how fast the Jets were able to ship Kane out of town. But the leafs want a big return for him, which I don't think they'll get, hence how things turned out with LA the way they did. LA wanted to swap Richards, and Toronto wanted LA to throw in a 1st and a prospect. Good luck getting a ~15th-ish pick and a prospect ontop of swapping bad contracts. Just not realistic.