The local BMW corporation negotiated how many vehicles they would be allotted from the available production capability. If either side miscalculated the demand (high or low), and someone doesn't cancel, they get built as the allotment slot comes up.
In some dealerships I've checked with in the USA, some have a vehicle or more in stock, ready for sale, some do not, and have sold their entire allotment. The dealers sometimes have a borrow/share policy, so what may not be available at one, might get traded to another. That's what happened when I decided on mine - my dealer did not have one like I wanted, but they were able to trade with another that did.
I concur with the comment about advertising...there has been very little advertising in the USA on the i3. Some of that is probably because in most places, the cars are selling without it. There are places in the USA (around the big middle plains area) where they have not been selling well, which, to me anyways, makes sense since it can be over 100-miles to the next town of any size - a city car, even with a REx, just doesn't make sense!
I think, but do not know, that BMW corporate opted to offer a larger allotment of i3's to the USA because we tend to buy more of the bigger, less fuel efficient vehicles, and the i3 sales tend to offset that and help keep the average fleet fuel economy down so they don't have to pay penalties. Another part to this is the general dislike (misguided in my view) of diesel engines, which tend to be the majority of sales most other places verses the gasoline versions. With our relatively cheap fuel costs, and loose taxes on things like CO production, most people don't want to or need to deal with diesel. Plus, while the sulfur levels have been mandated to be lowered on the fuel sold here, it is still higher than that available in the Euro zone. This means any manufacturer of a diesel engine vehicle cannot use the same engine here, and with the reluctance of most buyers to them, and the relative sales volume, they do not push many. So, something was needed to get the CAFE down, and the i3 is the hopeful venue.
Of the first four i3's my dealer had come in the door, two were already sold, and the other two were sold the first week...then, he had to wait for more. They seem to be pretty much going out as fast as they come in, and if you want something special, you'll have to wait unless you're lucky enough that they can find one near enough by to trade for you. Some people are willing to buy without seeing or driving a vehicle, but there's a lot more than will wait to see, touch, and experience the vehicle...then, with no advertising, it's the enthusiasts that are looking. Once more are on the road, I think the demand will peak, and delivery times will become longer until the factory can expand production to catch up. BMW did not expect the demand, and it takes time to buy new equipment and build space to expand.