One reason is they are hard to find.
When I went looking for a Club Spec it was like looking for a needle in a haystack. Oh sure they had plenty of those up a notch with air...
I received an email from a fellow out of the Mazda headquarters in Irvine, Ca. He gave me a list of 6 Club Spec's (without air) located in California. I finally drove the four hours after emailing and talking on the phone to the salesman.
The new and younger salesman (greeter) let it slip during the deal how they were going to be glad to sale this one...cause it didn't even have air! As the deal was as good as wrapped I told him with a chuckle that was why I wanted it and it was most likely the deal maker along with the 5-spd.
Now if it had crank windows, maybe window wings, and no radio I wouldn't have bothered holding out for a fair price for my 2002 base miata trade-in.