I guess it's because their governments want to stay in control of their citizens' transactions. And it's very difficult when people use cryptos instead of traditional banks.
Currently, the Chinese and Korean countries are forbidden to ico because of the safety of their people - this sounds somehow funny considering that most of ico projects are created by this people They are present in literally every 2nd list of devs.

And US citizens are blocked by many ico-s because US regulations are to strict and complicated. So it's easier for ico-s to forbid US citizens then to fit all their requirements.