Nothing really. You can say that it's because in week 2 you have to play three games in one day so if you have a problem to start it is difficult to fix, but that's not it. The problem is that people take a decent performance in week one, where the NA teams lost games against the Koreans and pulled out some clutch performances to win games they should have lost or were 50/50, and are surprised when that doesn't always continue into week 2.
Immortals lost convincingly to Longzhu despite having the best start they could have hoped for. They beat Gigabyte Marines. And they won a game they should have lost against fnatic. Immortals should have gone into week 2 at 1-2 . The difference between 2-1 and 1-2 is obviously just one game, but we act as though one is dominant and the other was getting smashed. Immortals week was very similar to the week that most NA teams seem to have at worlds. You get smashed by a Korean team and you beat the other two teams with varying levels of play. Maybe win a game you shouldn't and play decently against a far inferior opponent. Pretty typical stuff.
This is vastly different from how C9 and TSM's week ones played out.