Assuming you're talking about downsizing the image, the reason you would sharpen after the resize is because resizing, itself, usually results in some sharpening. That allows you to tailor any remaining sharpening needs based on the current state of the image.

Now, if you're upsizing the image, then a consideration is that sharpening often has some artifacts (halo effect) and so scaling up the image also scales up the artifacts. In that case, sharpening after the fact takes that effect out of the equation.

Having said all that, I still usually sharpen before a resize and just account for it. The reason is that I have the resize and save process defined as an action in Photoshop, so that's my last step. It works, especially with practice given a specific camera.