There were plenty of things that contributed to the downfall of the system, but I think these are the big ones:
Crap Marketing: I saw exactly 2 TV commercials for the Turbo during its height. And I was living in the Seattle area, not exactly a tiny market
Crap Box Art: Come on! When you placed Turbo boxes next to Genesis they looked downright retarded. I think box art alone drove plenty of people to the Genesis when given the choice.
Crap Pack In: Take this with a grain of salt. I really like KC, I really do. BUT as a packin to show off the system, not the best. Probably should have been KC and 1 other game. Maybe Blazing Lazers, and certainly Bonk later.
Even at 12 or so I knew these things were f*cked up and probably could have ran their marketing division in the US better than they did. They should have translated and brought over way more Japanese games, especially RPGs. That's what I wanted more than anything else at the time, and while we did have some good ones, there just weren't enough. They also should should have quit f*cking with the Japanese box/cover art, replace the Japanese text with English and leave well enough alone. There have been plenty of threads about this topic, so I'll leave it at that.