I have a Turbo Graphics and had the Turbo Express Portable (blew it out with a variable voltage adapter accidentally set too high... Still have the TV tuner though). I blame the quality of games themselves, not the art work.
Aside from the slight novelty of Bonks Adventure or hype of R-type, the games we played on the TG16 were pretty generic platformers, side scrollers, shooters or adventure games. They just didn't have the library to compete with the NES. Walking down the video game aisle of Toys R Us you'd see 20 TG16 games and what felt like 100 NES games. I have no memory of great game play from the TG16 and I don't even think we tried beating Bonks adventure, let alone care about Keith Courage, Splatterhouse or a lame racing game, Monaco something I think. Maybe it had good games, but we never crossed paths with them.