People stopped buying fixed power scopes and duplex reticles.

It's really that simple. If a consumer market will buy a product, manufacturers will make the product and take their money. When consumers aren't buying products, manufacturers stop making them.

Hunting is not the driving application for the overwhelming majority of firearms purchases in the last two to three generations. Hunting as an application has been proportionately shrinking for these generations, while firearms and accessory sales have increased. Simple hunting tools are not the common desire any longer. Most of these younger generations have also realized advantages of enhanced marksmanship tools for hunting as well. The berth between "I can make the shot with this tool" and "this tool helps me make this shot" is massive, and when pricing and quality has reached parity, consumers choose the advantaged product option, and eventually, nobody builds cars with crank windows any longer... Fixed 4x scopes USED to be considered more durable than 3-9x scopes, but after a few generations, we realize they really weren't any more durable, and 3-9x is more versatile, so consumers bought more 3-9x's than 4x's, and manufacturers slowed their manufacturing of 4x's.

We saw from the post-War era through the 1980's a tide shift in marketing; post-War, manufacturers, or rather their marketers, told consumers what they should want. And when everything on the market was new, that worked. But after a few generations of cultural experience with these products, consumers realized they have the power to tell manufacturers what they want, and after another generation, the manufacturers have realized their best option to survive in a fickle, capitalist economy is to listen to demand, and make what is desired. Multiple generations of consumers walked into those shops and told the counter workers that they didn't want a fixed magnification scope with a simple reticle, they wanted something with enhanced features - the shop listened, the manufacturers listened, and now that's what they offer. For a couple of decades, I had dozens if not hundreds of folks come to my door, asking me to build M4gery carbines - but knowing what I did, I warned them, it's extremely rare that civilian consumers really want a 16" AR Carbine with clamshell handguards, and certainly don't want to pay a boutique premium to have one built. More often than not, the folks which DID buy one ended up back at my door, asking for retrofits to make it more useful for what they actually do, OR asking for help reselling it, because they just didn't need the M4gery that someone told them they needed.

So it's really that simple - folks stopped buying fixed 4x scopes with simple duplex or G#4 reticles, and instead buy scopes with enhanced features, for the same quality and durability, at competitive pricing (volume advantaged pricing). A 3-9x with a graduated reticle can be used like a fixed 4x with a simple duplex, but the reverse isn't true. Can't blame companies for not making and selling stuff which nobody is buying.