Because pressing Start is so hard! Or L+R+Select, which is how you activate Sleep Mode in most games. And my point was, if Nintendo *did* allow 3DS sleep mode, that could cause weird things if the game was put into GBA sleep mode at the same time. GBA sleep mode already turns the screen, speakers, and CPU off.
And you can't believe that when Nintendo gives us 10 free games that they have no plans on selling to the public, they wouldn't spend a lot of money on making them? This whole thing is a loss for Nintendo, why spend a few hundred thousand dollars adding features that these games never had, when they won't be making Nintendo money, nor will many people be playing them?