For one, the main justification for belief in aliens is simply the vastness of the universe and that probability suggests that as it has occurred once, there is a very high chance it has occurred elsewhere. Meanwhile, justifications that I've seen for belief in a god similar to this are based off of the appearance of the universe to be designed and the probability that it was pure chance. The differences lie when you consider that belief in aliens is based off of the already known existence of life, but (this justification, anyway) belief in a god is based off of conjecture with a god as an explanation. There are a few arguments against the supposed design of the universe, but I'll leave it at that. Also, you don't see many people claiming that they believe in a god, but that they believe in their god. While the justification shown above could be considered evidence for a god, it says nothing for specific gods. And I recognize that many people have had experiences with their god, but testimonials don't do much for actual evidence and many people of different faiths have had these experiences. Meanwhile, the vast majority of people don't believe in any specific kind of alien, just the fact that life of some form exists out there.