Well, they were from the late 90's 'til about five years ago, when the import aftermarket started to lose steam. Now that segment has started to wither. Why? Same reason...no stripped down, affordable performance platform.
It's interesting to me that the v-6's the big three are putting in the base pony cars make very respectable power, but nobody is interested in tweaking them. One of my customers, a late model Ford tuner, developed a single turbo kit for the 6 cyl mustang and it was makin' 450HP with only about 15 psi of boost, yet they couldn't hardly give the kit away. You have to wonder if the v-6 has a stigma or something. I don't know why it would...the Buick 3.8L turbo used in the Grand National had/has a huge cult following. But still, I don't see any sort of aftermarket momentum for the 6-cylinders. Absolutely nothing comes through my shop....and if it's even a little bit popular, I'll see it.