The ocular muscles are 100X as strong as they need to be to control the movement of the eyes. I suspect that's Mother Nature's way of ensuring we can quickly protect our eyes from various dangers. So, by the time we DO have vision issues, those muscles are toast. If you look at it that way, it makes perfect sense that the weakness could be detected earlier, and more accurately in the oculo-facial muscles than those of our limbs.

Hope this helps.