Because it's better to bring it direct to us within an hour without horrific 20 minute long demonstrations, pausing for ridiculous American applause (which ends up taking 5 minutes total in a one hour presentation), buzz words, sales data etc.
Instead, it's game game game game game game game.
No TV TV TV TV TV TV TV TV TV TV TV TV TV TV TV TV TV TV. No buzzword buzzword buzzword buzzword buzzword buzzword buzzword buzzword buzzword buzzword buzzword.
It's about the games!
In April, there was a 37 minute Nintendo Direct, mostly focused on the 3DS. It contained over 20 games and services, all fully detailed. It's a far more efficient delivery system