NSA Whistleblower Edward Snowden: 'I Can't Say I'm Safe' in Russia – HuffPost

Another thing to keep in mind is that, at the time Snowden left, it was not clear how long protection would be nessasary. He might of thought that he would only need to wait out an election cycle or two, and public perception would have shifted enough for him to come back (I think it is still in the realm of reason that the winner of the 2020 presidential election will pardon; but the odds seem more remote now than at the time of the leek)

The other thing he has going is that I suspect a US president may not want him back as much as you may think. Neither side will be angry at the President for not getting him back (absent something blatant, like turning down a no strings attached offer from Putin); but prosecuting him would piss of a sizable chunk of voters.