Evans IS european based (he lives in Italy) so you probably meant to say European-born. In which case, why should that matter? Cycling is NOT a nationalist sport, and sponsors will go for people who will get them either a) wins, or b) gets them air times by enlivening races; neither of which Evans did with any sort or regularity pre-Worlds. Also, the way in the past he has treated his team in interviews, being whiney etc is not good for the image of the team or the team morale Heck, even Cav is more gracious in interviews than Evans usually was in the past. A combination of these things, along with very few teams with GC ambitions not having a GC rider already, meant that there were few options, and in hindsight, the right choice WAS to stay with Lotto; the issue was he had clearly alienated his team, which is why he left for a new team.
If he had stayed with Lotto, then he wouldn't have had to tire himself out singlehandeldy to get 1 result in the Ardennes (would have had team support, so could still have got the win without as much effort I mean), would have had the option of doing either the TDF or GIro as they would have had other riders, meaning he could properly concentrate on getting that 1win, and had better mountain domestiques to help him.
However, lets not forget that he has still had his most succesful year to date in terms of actually *winning* things (both a classic AND the points jersey in a GT), and ridden the hardest Gt course for years, one not suiting him particulaly, well to get 5th and the points.