I'm just guessing but one reason could be to avoid having to support foreign payment, tax and possibly warranty regimes (Jolla, the company behind Sailfish OS, is based in Finland).
Also they might want to avoid doing business in countries that support software patents and are over litigious over that (eq. USA).
But this limitation is pretty much artificial - the only thing you need to do from the EU is buying the image. This can be achieved via an EU based VPN (reportedly the built in VPN in Opera works) and AFAIK any mainstream payment card will work (does not have to be from an EU country).
Once you got the license and installation image, there are no real EU specific limitations. I have used Sailfish OS twice while on a trip in Japan and all worked just fine - repo access, etc. Also from the download statistics from OpenRepos (a community package repository for Sailfish OS), you can see there is a lot of users from countries that are not officially supported:
