Thing is, you're melting plastic and placing that melted plastic where you want it to be. In gravity and endless atmosphere this is easy, the gravity helps feed the raw materials through a hopper and ensure that the plastic stays where you place it, and the essentially endless atmosphere carries away noxious fumes so that you don't poison yourself. Unfortunately on a space station or in a spacecraft you have no effective gravity and a very limited atmosphere, so you cannot pollute nor can you rely on gravity to make things go where you want them.
Gravity making things go where you want them?? Gravity is a limiting factor in terrestrial 3D printers! The plastic is loaded as a filament and is mechanically pushed through the nozzle so no gravity required there, and without gravity you could get much better overhangs without requiring supports.
As for the fumes it would be pretty easy to have the printer in a separate fume cupboard if they needed.