Why not? Whay would you make a blanket statement like that? Have you tried them?
Just because S/S springs work OK in some instances does not mean Caltracs won't. Just about every competitive NHRA stock eliminator car runs Calvert set ups which to me is an indication that they work pretty well. Those guys don't just throw parts at their cars, everything is gone over to the nth degree. If they were not beneficial in some way, they would have come and gone.
Theoretically the main advantage of the Calvert setup over springs is their adjustability. If you read up on them, racers say they help making launches consistent and can be tuned more easily for track conditions. Unless I'm mistaken, there is no way to adjust prelaod with regular springs except when you add or a remove a leaf. If your car is not going straight, you can adjust Caltracs pretty easily in the pits. Have fun messing with leaf springs between rounds.
Most successful S/S spring cars have the front spring sections clamped. Caltracs essentially do the same thing (stiffen front spring segment) but employ a more of a triangulated set up than just clamping a spring pack. Running the Calvert setup eliminates the need for a snubber. They're also a LOT lighter than regular springs and there is no disadvantage to removing unsprung weight in a race car.
Some of your choice might be dictated by the weight of the car. If it's anywhere near 3,000 lbs, Caltracs would be a good place to start for a 10 second ride. Anything lighter you might want to consider a more involved set up. Caltracs are probably the least complicated and cheapest of all the options you posted above. Get their shocks too (front and rear) a set of 6 cylinder torsion bars and you'll be at a good starting place.
