What hurt the Turbo was not enough games, lame box art early on, and not enough advertising for the good games it did have. The early TV ads I would see in the morning were actually pretty cool, but they served more as an advertisement for the system itself and were too interested in attacking the Nes, as opposed to the Genesis which was the real threat. The individual games needed more ads too and NEC didn't seem to get that until Splatterhouse and Bonk's Adventure was released.
You can make the system look powerful all you want in a tv ad, which is even easier to do when you only compare it to the Nes, but unless the average gaming consumer started seeing lots of cutting edge, cool looking games being advertised they wont bite. Normal consumers were a lot less informed back then and relied on word of mouth, tv ads, rentals, and magazines for their game info.
Sega had a grasp of how it all worked here after the Master System fail. Nec of America though just didn't get it. They were too busy marketing the system like they would any of their other high end consumer electronics, focusing on the hardware mainly. They had a potentially superior product to the Genesis, but they simply forgot why. The games sell the system, the system doesn't sell the games. They didn't tap into that extensive Japanese library enough. The exact same library that crushed the Mega Drive.
Also, what could have helped a bit would have been Nec releasing all those Sega ports the PCE got over here in the NA market. Running advertisements in magazines or in a commercial of Sega arcade classics, Altered Beast CD, Shinobi, Outrun, Thunderblade, and Afterburner in addition to the Space Harrier and Fantasy Zone we already got, running on the Turbografx would have been a good way to blind side Sega marketing wise, in addition to looking good on a retail shelf.
Anyway, outside of the Johnny Turbo shit, the TTi crew did have a better grasp of things. They were just too little too late. You cant come along almost 3 years later and expect to drastically turn the tide, especially when you have no Mortal Kombat or Street Fighter 2 to offer up. It was unrealistic of them, but TTi did at least get some pretty cool games released here before things ended for the system in NA.