The thing is that the sonic isn't really being marketed as a hot hatch (or sedan). Even the RS "performance" variant is mainly for looks and doesn't come with any more power. The fiesta & focus ST, civic SI, Fiat 500 Abarth, mini Cooper S, GTI, and WRX are all performance variants of their pedestrian counterparts... in actual performance and marketing.
People shopping for a performance car in that segment wouldn't consider the sonic a performance car, and rightly so. Is it fun? Yes. Good handling? Sure. But the aftermarket isn't there because most people are just buying these as everyday commuter cars. Hopefully the support continues to grow as they get more into the hands of second owners who are looking to tinker with them.