Slightly offtopic tangent: So by now we're all aware that these are already ubiquitous in Europe. The question I have is why it took so long to reach the US? Why didn't the main shipping companies there like FedEx and UPS step up and use their existing infrastructure to build something like this, much more easily than a startup could do.
That is, in European countries with strong labour laws and unions, it's relatively expensive to pay people to deliver packages when most people are at home (early evening hours). The obvious answer is something like the Packstation, replacing labour cost with machines.
The US has a much wider market for very low income jobs because the marginal cost are lower, thus US companies can make money with jobs like people guarding parking lots, something you hardly see in Northern Europe (the weather might play a role, though...).
So naturally, such solutions spring up earlier where the relative win is higher. Plus as others mentioned the denser/more urban life style.
That's not a clear win by the way, lots of relatively decent jobs for low skilled workers are lost to these machines. The machines are cheap, but it might still have been more efficient to pay slightly less to the delivery workers (and kept them in jobs).