Scotland Calls for Mandatory Vitamin D

Here's an article from The Guardian about the link between a lack of  vitamin D and MS. In Scotland, which has one of the highest rates of MS in the world, they are calling for mandatory doses of  vitamin D.

I don't know if vitamin D helps or not, but I do take at least 1000 units per day. There certainly seems to be a valid link between lack of vitamin D and MS. Why is it that it is an illness found mainly in countries with little sun in winter and nearly unknown in countries  that  get the most daylight hours?