Not sure that the premise of this article is even correct. I can think of a lot of successful European startups -- Minecraft, Spotify, Skype, DeepMind, the list goes on... -- and just as many startups led by Europeans that have succeeded in America (Lending Club, etc.). But if Europe has trouble supporting "unicorns", there are probably a couple reasons: 1) Gaps in the funding ladder. 2) Talent flight. 3) A preferenc…
The article also doesn't mention an obvious bias, that is, if a startup starts in Europe, there are a lot of incentives for that startup to move to the U.S. early in its life cycle. In that scenario, the startups who become successful (and who originated elsewhere) all get counted as U.S. successes. This would vastly undercount both the total number of startups elsewhere as well as the number of successful startups.
The question that still stands is why they are moving the the US.