The lights are on
I'll go for two main reasons - processing power and online functionality
I own DS Lite and 3DS and it's noticable technological gap between those systems. It may not be obvious at the fist look but after spedning some time with 3DS and it's games it's easy to notice the amount of power in this little devil.
SSM was always focused on multiplayer (despite solid sp mode) and the old DS simply can't deal with all the streetpass, mii and dlc (my guess) functionality. Also online play on 3DS is better comparing to his older brother.