Well I could be full of BS here, but I think its partly to do with where and how its mounted, and airflow.
Radial engine is designed to be mounted flat on a firewall, with cylinder heads projecting out for cooling. Its basically round, going on a basically round fuselage. And the output is connected to a prop so simple, easy, effective power transfer. Oil will drain into lower cylinder heads on most models that I've heard of, so the prop needs to be "walked" around before starting.
Now lets look at a car.
You can't have it mounted like on a plane in front, or you wouldn't be able to see over the cylinder heads.
The other thread showed a small one mounted on a cycle, but it really isn't ideal, as the airflow is 90 degrees off for optimal cooling. To put that size of a radial in a car would be possible, but you'd still have a very large hump to try to see over.
On a car it would almost have to be mounted horizontal, the drive would have to go through a 90 degree change of direction, with the resulting loss of power, efficiency, etc. Airflow would have to somehow be ducted around each cylinder head from the hood, down and around. Its the exact opposite of the same motor mounted in a plane. Lots of problems, lots of issues with the drive shaft, power train, etc.
One option would need a body shaped almost like a plane, with 4 very tall wheels. Driver is perched up above like in a plane, to see over the top. Even then the drive shaft comes off the wrong side, and still somehow has to cut right through the whole works to reach the differential.
The only way I really see one working, is as a rear mounted engine, directly coupled to a generator. You won't get as good of cooling as you do in a plane though, because instead of being mounted directly behind a huge fan, its mounted behind a generator. But power from the generator could run electric motors in all 4 wheels. Even then your going to have airflow and heating issues, as some heads will be getting more air, and thus running cooler than others. But noise and and drive train issues would be better.
Radial engines are a specific solution for a specific application, and really are not practical anywhere else.
