No one would pay to have a 3D printer fixed. Or at least not pay enough for it to be a viable business. A brand new printer can be purchased for the equivalent of a couple hours labor, and that’s before replacement parts.

Customers may buy a retail functional replacement now for <$400.

It is called the Shoemakers paradox, where the shoemakers kids go barefoot.

Also, the same reason why CNC Milling factories don't tend to produce paperclips. =3

Rule #23: Don't compete to be at the bottom, as you just might actually win.

Not enough business, not highly paid enough. No true market for such a service.