I have a theory.
We have one or two of these things for the eight year old. They were fine as a sort of introduction to gaming, but she quickly got bored with them. Especially since our house sports full blown emulation, PS2, Wii, Gameboy Advance and her idiot father just bought her a Gameboy DS and a cellphone. Makes it really tough for these Plug&Play games to compete with full blown systems with far better graphics and interaction.
The P&P stuff reminds me a lot of those old Game & Watch games or those old Tiger Electronics games that hit the market and everyone had to have one or two... and that's about it. Everyone bought one or two and the market wasn't sustainable afterward. I've got a couple of the old G&W games and one working TE game left and despite their low cost and easy appeal, they were easily eclipsed when I finally saved enough to buy full blown consoles and portables.