There are a lot of problems related to hosting a major in the US, even tho I agree with the idea of having a major in California would be awesome.
First, there are timezone problems. Organisers aim to get the most viewers from the event, and EU is the ideal spot for that.
Second, VISA issues occuring lately. If a team from some low tier country qualifies to the major, there is a high chance that their VISA’s applications will be declined, therefore the team will be replaced by another one.