[What is naturalistic? I suppose most linguistic universals are universal cos there happens to be no counterexamples, not because they were impossible.
That way of posing the question implies that there are only ways to explain linguistic universals; (1) that a certain feature is logically necessary, or (2) that it just happens to be the case that all languages has that feature.
Explanation (1) fails - at least for many universals - because those universals are statistical rather than truly universal. Explanation (2) also fails, because it would be highly improbable that, for example, 98% of all languages just happened to share a feature.
Thus other kinds of explanations are needed to account for the fact that many linguistic features, though not truly universal, occur among the world's languages with a higher-than-chance frequency.
Suppose languages could have either feature A or feature B, but that feature A is vastly more common.
This could be explained in the following ways:
(3) While both A and B are logically possible, A may be easier to process or something.
(4) While both A and B are logically possible, and also equally easy to process, B only arises given certain very specific conditions. A would be some kind of default, which all languages have on their deepest level of grammar. B may occur as a surface realisation due to certain transformational rules, that work only under certain specific conditions.