Because they needed something better than ds lite.
It was invented because it was an upgrade and it comes with a more open selection because you couldnt get this on the other by internet or game so the only way was by creating a new ds with added features
Because it is fun and portable better than the psp
Because psp was more advanced than the ds lite and sony was selling them for 160 bucks so nintendo had to make something better and sell it for the same price.
Because so wew could play and there bothe different prices the 1 with the camere is more expensive then the dsi