Because of the amount of R&D involved to do a fully enclosed setup. Add that to how recently the car's come out and it's unlikely we'll see it anytime soon, if ever. If you're coming from other markets, keep in mind the KDM scene is absolutely miniscule, so R&D and production costs can't be easily offset. The Gen Coupe had a huge market and there was only 1 fully enclosed setup made--by a rather small specialist-type company--and that was upwards of $500 for a single side. From my POV the chances are extremely low, that's all. If a big name manufacturer does step up, then it wouldn't be until Q3 at the earliest IMHO.
I believe they have their own mufflers. We're a Stillen dealer and I'm not a big fan of the company or their products. We actually had a recent issue with an exhaust and the customer was getting very little support and shoddy communication from them in regard to the warranty replacement, which really put us in a precarious position. Either way, don't get me wrong--Stillen is ok, but I just feel for their price range there's generally better overall options. Still though, it's certainly worth giving them a chance to see what they bring to the table for the Stinger market.