The question has to be asked, why do some people think that coil springs will be an improvement on the proper rubber cone springs?
Could it be that those selling the coil spring kits have advertised that they give an overall improvement? Those vendors just want the money/profit.
Work out how, for road use, they could be better.
On a race car, such as Nick's, they can make sense because the tracks are totally smooth. Find me public roads as smooth - you can't.
,,,,,,,"the tracks are totally smooth"........
You never raced at Llandow, or even Brands Hatch in the 60's !
And the answers to your opening question are:-
1) Because they are ignorant.
2) Because they are gullible.