There are various combinations of why you are unlikely to get a dedicated sonar hardware:
1. Sonar is not a hardware company. Many forget that it was Digidesign's hardware that pushed their product protools into prominence and not the other way around. Presonus, a hardware company has taken the plunge into software and even they have made the choice to make hardware usable by most.
2. More and more hardware works fairly well with multiple systems, making the market limiting dedication to one product less advantageous. The nektar keyboard controller works damn near as good as a proprietary device for multiple daws.
3. Sonar is more of an all arounder DAW than a specialized product. A product like Ableton gains far more functionality with a dedicated controller than Sonar or most major DAWs. The most common functions to a DAW are available on any decent control surface. With the nektar controller ( a relatively inexpensive option) I can create new tracks, arm, record, select tracks, control vst instrument settings, control effect parameters and so much more with minimal setup. Ableton, being more of a performance platform than traditional DAW requires it to have needs not really covered by generic controllers.
4. Sonar simply doesn't have the demand for such a product. For many sonar users having such a product may not do much more for them than current setup. When Roland owned company, some purchased controllers that were designed to work well with sonar and results and response were underwhelming. Not dismissing product. It was actually decent, but not so good that it was viewed as must have to enhance sonar experience in way that was significant.
5. The idea that Tascam division of Gibson could somehow create such a product seems farfetched, considering the struggles it has had in recent years. Big roll of the dice for a struggling company to do for a product that may not have the demand necessary to warrant such a risk. Besides, when companies like Gibson acquire companies like sonar and tascam, the level of integration rarely becomes complete enough for fortunes to merge so tightly.