I think it's because people are finally discovering what a wonderful little system it is. Back in the 8-bit era, most people missed out because of Nintendo's bullying of retailers and publishers. When you corner 90% of the market with a monopoly, it's hard for other machines to get an audience.
The GameCube wasn't the same situation because Nintendo's hold on the market was long gone by then. I guess most people didn't have room for a third machine, and now that it's cheap, they're finding out that the GC was a great console.