and therein lies the problem.
HIIT has actually been around and been discussed in running groups for a number of years. Lest you think I am pulling this from where the sun doesn't shine, I write this from some personal experience; I am an experienced ultramarathoner (six 50 milers). HIIT is extremely difficult for "normal" people to do as an ongoing exercise program.
The great majority of Americans are simply not capable of pushing themselves as hard as is required for a successful HIIT regimen. If you're not capable of pushing yourself to do this type of strenuous exercise, you're not going to do it. It's as simple as that.
HIIT will work extraordinarily well for people that are already moderately fit or even overweight if they are capable of pushing through their pain (not the physical pain, the mental pain). Again, and again, and again; and each iteration is harder than the last.
Most people - especially the great unwashed overweight masses (pun intended) - aren't willing or capable of doing this, and simply aren't going to do it. They would be better served starting out just walking briskly for 30 minutes four or five times a week.