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Because the third party company that hosted the DS and Wii servers was bought out and the new owners shut down all their game servers, and unlike PC games that were effected by this, DS games can't be officially patched to change the server the games are coded to connect to.
Pretty sure there are fan-run servers for several DS and Wii games you can access with DS flash cards or a modded Wii respectively, but barring Mario Kart Wii I don't think any of them are well populated.