Here is my thinking...I do not know the details, but I did hear that California extended a tax breat to Disney if they spend a certain amount out there over the next 10 years or something, I really do not know the details and I have heard that they bought more land out West, so I am certain that they will have no trouble meeting their commitment (it may even be a 30 year deal)...so my thought process (again, I really do not know the details of the tax break) is that Disney, as a company, would look to save from said tax breaks by having the West Coast park bear the lions share of the design for this new land and as such, they would want to be certain to get started out West first.
Again, I am being a bit cynical, but it makes sense for business to take advantage of whatever breaks they can get.