For the product to provide long-term preventive sealing, it needs to remain in a fluid state. Driving a car on the highway with a tire full of liquid is dangerous.
Fix-a-flat style repairs are intended for emergency use only, just to get the car to a place where a real repair can be done, and usually are intended to solidify, not stay liquid. You're supposed to drive to a shop and get a plug or patch right away.
Other possible reasons for the difference:
The off-road sealant may have ingredients which could cause deterioration of the tire over the long term, something that might be acceptable for an off-road vehicle but not on the highway.
It could be a legal restriction, such as a chemical which is legal for off-road use, but not on highways, or some sort of tax or fee being included in the price of the on-road product, just as off-road diesel is cheaper due to not including taxes.
Most modern cars have some form of TPMS, and companies advertise their on-road products as TPMS-safe. I doubt the off-road goop has that assurance. Aside from TPMS, using a product not intended for highway use could void the warranty on your tires, or even affect the warranty on the car itself.
The whole idea of tire sealants for automotive use is that you use them as a temporary repair in an urgent situation just until you can get a proper plug or patch done.
The manufacturer also has to be careful of liability. If they don't state that the product is for off-road use only, and something bad happens on the highway as a result of their product, they're in trouble. At least this way they have the defense that the customer used the product incorrectly.
I strongly recommend you avoid using a preventive sealer for a street automobile.
The sealer either works or it doesn't.
If it works, you could get a hole and not even know it. Now you're driving on a tire that needs a repair, but you don't realize it. Your tire could be structurally unsound without you even realizing it, because it never leaks.
If it doesn't work, then you've still got a leak, but now you also have a mess, and the tire shop will be annoyed when they remove the tire and get that goop everywhere.