Well, it turns out that the early universe went through a phase of incredible, extraordinary expansion called inflation. The inflation was boosted by something called an inflaton field, and due to quantum properties the inflaton field could spontaneously destabilize and spawn a bubble universe, each of which with a different Calabi-Yau manifold. It's possible that we are inside such a bubble universe, and outside of us the inflaton field is continuing to expand and producing more bubble universes. If this is the case, then it's not much of a surprise why our universe has the properties it does. Out of the 10^500 possibilities, a bubble universe spawned with the right Calabi-Yau manifold for life, and (unsurprisingly) we find ourselves living in that universe. This is the landscape multiverse theory. (I originally read about this theory in Brian Greene's excellent book The Hidden Reality, which describes many other multiverse theories too.)
Now, something important to note is that all this is still highly conjectural. String theory has many pleasing theoretical properties, but about zero physical evidence to support it. What then is the difference between string theory and deism? The difference is that for string theory (and more generally other multiverse theories) we actually have physical and mathematical theories that describe them. Deism doesn't. For string theory, we have papers, conferences, books, equations, and physicists and mathematicians who study this problem. We have something to work with. What do we have for deism? Well, pretty much nothing. No data, no theories, no anything. Deism is a fall-back theory when nothing else works out, and it has no theoretical roots and makes no predictions. This may change in the future, but as of now, deism is a scientific dead end. Of course, I don't know if string theory or another multiverse theory will pan out (indeed, the failure of the LHC to detect supersymmetry is raising some eyebrows), or even if deism actually turns out to be correct. However, on a scientific basis, string theory has more going for it now.