One thing I have been puzzling about recently is why utilitarianism has become so popular among philosophers in academia who are doing ethics. One possibility is that there's an idea (which does seem to be floating around, although I can't vouch for its being prevalent) that utilitarianism is somehow more naturalistic than alternatives. I don't understand why anyone would think this, though. There is nothing intrinsically naturalistic about utilitarianism. The first utilitarian of note was William Paley (most famous for his design argument for the existence of God), and most of the early utilitarians were Evangelicals. The better known Bentham-Mill line of utilitarianism was part of a secularization of this.
Further, there are clear cases of naturalistic alternatives to utilitarianism; sentimentalism, such as one finds in Hume, would be an example. And the big rival of utilitarianism in nineteenth century Britain, the sort of deontological intuitionism one finds in Whewell that looks back to Butler and is influenced by Kant, seems to admit of naturalized variants. Indeed, if by 'naturalistic' we mean 'reasoning in the manner of the natural sciences', the whole start of of philosophy of science as we know it, which begins with Whewell and Mill, develops (in part) as an element in the battle between these two rival ethical views: both Whewell and Mill, while interested in scientific reasoning itself, are also explicit that they are strongly interested in marshalling this scientific reasoning, which has yielded such progress in so many areas, for the purposes of advancing ethical reasoning. And they do exactly this. But the point I wish to note in particular is that Whewell's arguments on this score are far more sophisticated than anything Mill ever produced. Whether they were more right is a tricky question; but they are there, and they are formidable. So utilitarianism doesn't seem to be inherently naturalistic; and it doesn't seem by any means to be the only promising naturalistic ethics. So naturalism wouldn't be a particularly good reason for focusing on utilitarianism rather than, say, sentimentalism, which is (in my humble opinion) a somewhat more formidable ethical theory.
