I must say explaining the distribution with biology reminds me of the ideas of the age of enlightenment, something that the likes of Rousseau would have thought. Today we should look for less biblical solutions.

As statistics has been mentioned, we can make the (universal) observation that most sentences have a subject, but far fewer have an object. The conclusion should be that the subject is (statistically) a more fundamental element than the object.

Now let's look at word order in WALS:

Subject-object-verb (SOV) 		565
Subject-verb-object (SVO) 		488
Verb-subject-object (VSO) 		95
Verb-object-subject (VOS) 		25
Object-verb-subject (OVS) 		11
Object-subject-verb (OSV) 		4
Lacking a dominant word order 	189

We see that all three SO systems are more common than all three OS systems. 

As the subject is clearly more frequent in the sentences of the world, it also seems that it appearing before the object is clearly more frequent in the languages of the world. Having a language that starts sentences with an object (talking of OSV) is perfectly plausible logically, but when you actually use the language you'll find that there will be just as many sentences starting with a subject due to situational lack of object. This puts pressure onto starting sentences with a subject anyway ‒ and so we see that it's precisely the resulting SOV that is the most common word order, although SVO is not far behind.

So what is so special about these two? Today we have optimality theory (OT) to study that.

Looking at optimality, one thing is that unambiguity is desirable (not obligatory) in a natural language. When you start a sentence with a conjunction and place the verb last, these two form natural brackets closing the sentence. You can test this by marking the conjunction with [ and the verb with ] as in (SV):

[If the sun round is][then the Earth round is][and we can around it travel][if we want]

This language has a kind of built-in sentence structure analyzer.

Another type of bracketing occurs in SVO where you can place an SVO subclause between the subject and the verb. The subclause has its own verb, but the appearance of the main verb also marks the end of the subclause:

My son [who is a great student   ]goes to school in Oxford

But VSO is not as useful in bracketing because if a subclause is placed between the subject and the object, and that particular subclause lacks an object, then it might seem as the object of the main clause is the object of the subclause, or vice versa:

Kicks the girl who sings a ball

VSO is however a completely plausible word order, and a VSO languages may disambiguate in other ways.

Of course thinking of all this will make your head hurt much more than Chomsky's simplistic explanation that evolution (or creation) accounts for the statistics...