One way to look at this is market potential...England requires right-hand drive, most of the rest of the world, left-hand. England's population is around 63M, the USA, around 317M...they started delivery of the i3 in the USA earlier this month...it's been available elsewhere since last year. There are more on the road, and more interest for a product is built up based on people actually being able to see and touch the things. The whole supply/demand thing will get worked out, but BMW is selling more than they thought they would. It takes awhile to order, install, and be able to use more material and expand the production lines.
I wouldn't be surprised that the wait in the USA increases as people start to see them on the streets and the US car magazines get production reviews of USA spec vehicles (not that there's all that much different), rather than prototype or non-USA spec vehicles.
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)