Because at the World Cup - real hockey with the best representatives of the game. Moreover, the national teams without injured players, only after training camps. The World Championship is a laundering which, due to the annual holding, devalued the significance of the tournament to the utmost, making it a tournament for losers, that is, those who lost in the nhl
Because the World Cup is a commercial tournament held by the NHL with the permission of the IIHF, and, accordingly, the NHL is interested in the best players in the world to play in the cup for greater profit. The IIHF itself is hosting the World Championship, focusing on the completion of the European and KHL Championships. If they were waiting for the end of the Stanley Cup, then the championship would have to be held at the beginning of summer, when the period of vacations and exams begins, and many will not have time for hockey. Plus, European players may lose their shape during a month of waiting for the tournament.