Why are first year of production classics the most valuable?

Looking at nearly all classic motorcycles, you will notice that a first year of production run of any model is the most expensive and hard to find. A Kawasaki Z1 900 from 1972 will cost 50% more than a Z900A going 1974-1977. A Honda CB750K0 will cost...

http://www.themotorcyclebroker.co.uk/why-are-first-year-of-production-classics-the-most-valuable/