Leaving more than 2 weeks for the world first race is more a safety measure. While the initial raid has historically been quite short there is no telling whether it will actually end up being ridiculously hard, or have some boss that is overtuned or final boss that requires clever use of game mechanics, in which case essentially forcing raiders between progressing or taking a holiday break would be a pretty terrible move. Far safer to just delay the raid release until after the new year.