at one point the makerbot replicator dual, were the most copied 3d printer on the planet. (way way back in the dim an distant past - about 6 years ago :-) )
Then josef prusa invented the i3, and gradually copies of that became the most produced fdm machines around.
It's just really really easy to make. Particularly if - like the ender 3 - you throw away all the best aspects of the original design.

The rep clones are still around, and have been constantly improved by the chinese companies that make them. They Just have not increased the print volume.