It's a lot of work to set up billing in all the different countries, so they may not have bothered. There are also a lot of legal issue with different privacy laws (Europe being stricter) and censorship requirements (e.g. swastikas in Germany, denigration of Attaturk in Turkey). It's not likely that anyone's actively avoiding global availability, but it may be more worth for a period of time to use their limited resources on making the US version better rather than dealing with those headaches.

When it's fundamentally broken internationally and people start giving it bad reviews when it doesn't work. That's the only case I can think of and even then it's quite weak.