Because most of the people who spent big money on a-series engines needed to
be legal for a race class and these heads aren't
It's expensive (~US$3,500 complete with extractors etc) but much less than a
supercharger and the powerlift (HP and torque) is pretty impressive if the
numbers match the reputation.
The question I have is why are they not more common? They've been around
since the 60's. Why if they are so strong has someone not long ago mass
produced them and brought the price down?