What really stops spin trainnnig for pilots is instrument training.
Today, FBOs want to be able to use ALL their planes for instrument
instruction. It no longer makes sense to have different aircraft.
Spinning an instrument training aircraft is a really bad idea because
it bangs up the gyros and increases maintenance costs on the gyros. I
would love to give my students spin training in the Cherokee but the
FBO doesn't allow it because its an instrument platform too.

-Robert, CFI