Because, you know, they totally didn't already do this years ago, right? The Super Game Boy and Game Boy Player weren't actual things, right?
Here we go again... Nintendo releases something cool, and people forget about it (or even mock it) over the years. Then Sony releases something similar (sometimes years later) and people are like "DURR, Nintendo is so out of touch, why don't they do this??"
Back on topic though, I think I might actually grab this thing, eventually. There are a small number of Vita games that I do want to play, and while $200 on a Vita is a bit much for just those few games, $100 seems a reasonable price, at least for me.
Besides, playing PSP games on a big screen (without having to use an annoying cable) would be pretty cool.