Bahramthered, we have an oxidizing atmosphere and extremely aggressive bacteria (as well as a fairly complex chemical environment). Organic compounds tend to break down unless protected in some way, and bacteria seem to be everywhere we look.
So, for new life to form it would have to be in a low-to-no oxygen niche, and it would have to be totally uninteresting to bacteria that are in that niche, and any molecules that are around the new life have to not destroy it.
That doesn't prevent new forms of life from forming, but it does narrow down the places where it could form.
But, what do I know ... I'm not a biologist.