It is not a technical issue, and the money aspect is secondary. It's a regulatory issue. Every tire manufactured for sale in the US must comply with the NHTSA regulations for vehicle tires. One of the requirements is the tire manufacturer cannot sell a tire for a particular use unless they have tested and certified it as suitable for that particular use, hence the letter P or LT, meaning Passenger vehicle or Light Truck in the tire designation. The words on the side of the Kenda do mean something. They mean the tire is designed and tested as suitable for use on a Can Am. Same with Slingshots. Suitable for use is why some tires are labeled "Not for highway use" or "Special Trailer".
It's a money issue in that I'll bet most tire companies do not see a sufficiently large market with Spyder owners to justify the cost of certifying their tires for Spyder use, so they don't market them. What we do not know is if there is in the archives of the major tire companies test data or engineering analysis that shows car tires really are not suited for light weight vehicles. Our experience shows CTs work very well, but none of us has the expertise or test data (not usage data) to unequivocally prove that CTs work great for Spyders. And for all we know BRP may have engineering or test data that shows CT are, or are not, ideal for Spyders. And you can be sure when the Spyder was initially being developed it was a lot easier to let Kenda tackle the engineering of tires than for BRP to do it. The big name tire companies probably wouldn't give BRP the time of day let alone seriously discuss providing tires.