The big problem for SYW plastics is that there is a profusion of uniform variety within armies. They are all basically similar, but differ greatly in detail. The Russians are the most homogeneous followed perhaps by the Austrians, but the likes of the French and Prussians have so much variation (collars/no collar, lapels/no lapels, different cuff styles and so on), that you would have to compromise on accuracy.
If you could solve those issues, you then have to remember that SYW is still niche compared to ACW or WW2, and you have to sell boatloads of plastics to make it viable. Plastics tend to be for subjects that you need loads of troops for. For Naps you sometimes see more specific troop types, but the period is so popular (relatively speaking) you can still sell a lot of, say, Chasseurs a Cheval.