Include an extension in your template name, including ".twig" (e.g. robots.txt.twig)
This is because right now, if your template path includes an extension, Craft assumes that you’re explicitly providing the full template path, and won’t try appending the defaultTemplateExtensions to it. (See a full explanation here.)

robots.txt does make a good use case for why we should append those extensions anyway, though. We shall consider it :)

UPDATE

We changed this behavior in Craft 2.2, so now a request to /robots.txt could resolve to a template called /robots.txt.twig. http://buildwithcraft.com/updates#build2579