What is the reason of the disparity between single article price and yearly subscription in scientific journals?
On the principle a subscription is always better than individual sales from the journal's point of view because it's a fixed income on which they can count. It's also well known to increase sales that explain why you always get a discount when you subscribe to a service.

As for the seemingly large difference you mention, my guess is that individual subscriptions for scientific articles are rare, and thus not very interesting financially. People needing frequent access to the content of these journals typically access them via institutional subscriptions.