Ease really. Once the patents expired it was fairly simply to make a 3d printer, which is essentially a hot nozzle squirting plastic. It’s more akin to a pen plotter

An inkjet printer is significantly more complex. Even if you know how it works you essentially need a semiconductor level fab to make the printhead. That’s why all these projects just reverse engineer hp cartridges. Literally every inkjet printer patent could’ve expired 10 years ago and it wouldn’t change the engineering and manufacturing challenges