Either there were problems with the kind of deal the Canadians gave Halak, and he wanted out and they have faith in Carey Price. Or they have access to the best crack in Canada.
I don't think there's anything necessarily wrong with dealing Halak. They'd be selling while his value is very high and Carey Price is still a very promising prospect. Montreal set themselves up to make a decision like this when they brought in Gomez' and Gionta's contracts this summer. They're too far up against the cap to afford both goalies, and probably figured that Price+Plekanec is better than Halak+whoever they can find in free agency to replace Plekanec. The thing that made no sense to me is how quickly they traded him. They didn't qualify him, didn't allow the negotiating team to reach out to Halak's agent to discuss a new contract, didn't play offers against each other. They didn't even wait until the NHL draft, when teams start to really make their moves. All of that significantly reduced what they got back. Ellers looks to be a good center prospect, but they should have been able to get more for one of the game's best young goalies.
sigh, the western conference just got tougher. I don't think the blues are contenders but they're definitely poised to make the playoffs now and will be a darkhorse team when they get there.