Two reasons:
1. Everyone is sick of the ASG. The fans have kind of soured on it, the players frequently skip it, the NHL needed something new to bring excitement back to the league.
2. The Olympics are run by the IIHF, not the NHL. The NHL gets nothing from allowing players to play in the Olympics. They actually lose money for those weeks. So this is the NHL's way of having an international tournament, as well as sending a message to the IIHF that they may not be needed in the future.
Also, extra bonus reason: This tournament is played under NHL rules, and thus, allows things that the IIHF wouldn't. If an Olympic game started with 3 fights in under a minute, every one of those players would have been kicked out of that game, and probably would have been suspended for the following one.