Well, part of the reason that happens, is because every firearm manufacturer I know of uses batch production.
They can't keep every model in continuous production. So, they have to make an educated guess about what the market will do between scheduling (and ordering materials), the intended run, and the next future run.
Sometimes, they miss the mark a bit, a competitor comes out with a model that eats them for lunch, or the economy tanks. It's just the way batch production works.
And... they base their estimates on distributor orders. If the distributors are padding the numbers, already, then it may lead to a significant over-supply.
It's just the nature of the business.
Until some one creates a company with a feasible, affordable on-demand production method... we'll always be buying firearms that are 3 to 9 months, or even 8 years, old.
"Such is the strange way that man works -- first he virtually destroys a species and then does everything in his power to restore it."