There weren't good CAD tools to send the designs to the machine until recently. Early CAD tools mostly produced drawings and it was a separate step to manufacture from there.

The way inkjet and laser printers work is also quite different from the way a 3d printer works. The similarity is mostly in the gantry, so there was nontrivial innovation required here.

To some extent 3d printing is probably also a reaction to decreased access to domestic manufacturing. It doesn't make a lot of sense to produce most parts in plastic if you can get a cast or milled part quickly and cheaply.