ACOGs are great optics, but with as many more reticles, colors, and configurations than there are world religions, it is hard to stock a wide variety of them without investing your liquid cash flow into an overhead item that may take a long time to sell...
That is why places like Element usually just stock the most popular selling models - to lower the amount of time their $$$ is tied up in overhead.
If you stock a less popular, niche optic, and it sits on the shelf over 1.5 years, the manufacturer may come out with a new model, and now the going market price of the older generation niche optic has dropped significantly - possibly even lower than the original dealer cost of the item, and there it sits, unsold, and if it does sell, it will be at a loss. Not smart business move.
The most popular models are usually popular for valid reasons, and they will sell faster, and make a bit of a profit instead of a loss.
That said, ACOGs aren't the be-all end-all of the optic world... There is a lot of hype that goes with them. Depending on your intended usage, there are likely much better options available.