Most of the information provided is from personal experience so please take it with a grain of salt.
There are at least 3 reasons I can think of why they wouldn't use IR, in most cases.
First is the hot mirror. CCDs in your camera probably ARE very sensitive to IR light, however all cameras come with a preinstalled hot mirror that reduces IR light from reaching the sensor. This is because IR is outside the normal visible spectrum and basically if it's in a normal photo it gives the edges a soft look (check out full spectrum photos for that effect) As this is considered 'undesired' data they block it. And as someone posted it appears as Chromatic Aberration. So using an IR light to help focus would have reduced feasibility unless it was really 'bright'.
Second, the IR spectrum is a very long wavelength, and so the focusing mechanisms in your camera, which are calibrated to the visible spectrum, will be a bit inaccurate. I believe, the closer the focal length the further off it is. This page describes it best http://www.lifepixel.com/focus-calibration-options so I'll leave the science to them.
Third, is interference. Many speed lights/remote triggers/etc use IR to send information from each other or to the camera, with specific encoded messages in the IR range. If you were to use a focus light while, say pointing an IR remote trigger at your camera while standing in front of your camera. Well you'd potentially drown out that remote's message, and the camera wouldn't be able to accurately tell when to actually take the photo.
Finally, I believe the reason they use red light as opposed to blue, is that it's the least invasive of the visible light bands. I guess that's subjective somewhat though.