Why not deism?

[Objectivist]

First, we should ask, "Why deism?"

What reason to we have to think there's a god of any kind in the first place?

If we want to really get down to an ontological principle, we should ask, "What is a god?"

That might be the fundamental problem with deism, actually. In that case, though, atheism loses cognitive meaning, as well.

The issue is basically, why is there something rather than nothing? That is at least a reason, if not an entirely convincing one (and to be convinced is subjective anyway), to accept some sort of necessary existent.

"why is there something rather than nothing" is an improper question.  As soon as you offer a reason you are talking about existence.  You can't explain existence by pointing to something that exists.  You'd have to step outside of existence to look for an explanation.  But if something doesn't exist then it can't explain anything.  

If an answer to the question of why anything exists is the basis for your belief, then you're in trouble. You don't get down to an ontological principle by starting with an error.