Why aren't 998 and 999's more popular?

I think they are a 904 (in a 904 case) with 5 clutch packs instead of 3. Why aren't they more popular? Maybe they aren't as plentiful as they would have been an option. I don't know about the gearing in them.

I have one in an M body that I plan to put in my Duster. Still quite a few old M body cop cars with 998/999 in them.
C