Because of . The 3DS has a built-in DS processor. If you install Custom Firmware (CFW) like Luma3DS, you can run original Nintendo DS ROMs —including the DS’s firmware—directly from your SD card. The real PictoChat lives inside the DS’s operating system, not as a cartridge.
: It uses local wireless (LAN) rather than the internet, meaning you must be physically near other DS/3DS users to chat. Pictochat 3ds Cia