Why might it be that one have impaired vision?

Long list: The most common is a need for eyeglasses to see clearly. There are any number of pathologies that can impair vision including corneal disease, cataracts, uveitis (internal eye inflammation), retinal detachment, diabetes, optic nerve diseases like multiple sclerosis and brain problems like strokes in the visual cortex. Check with an ophthalmologist to determine the cause.

Many reasons: This is what we have eye specialists to investigate. I would recommend an appointment with one.