An awful lot of systems are still setup to use carbon copies as the method for generating the 'official' copy of a document. If you want to print onto a form that uses carbon paper, you have to use an impact printer.
A market need still exists, it's unsurprising that someone is filling that need.
I think part of the surprise is not that the need is being fulfilled, but the fact it is a need at all.
Never underestimate switching costs. For the kind of organisation that plans IT purchasing five years in advance, a $500 dot matrix printer is a bargain.