That and Nintendo LOVES BC (unlike the other two companies). Yet another reason why the DS (and Wii) sold so well - 100% BC with the older generation.
I love that I can play GBA games on the DS. I think I've used my GBA slot for about half my gaming on my DS.
But, eventually, they do lose it. The DS is not combatible with the old GB games, for example (though the original GBA was, IIRC, but later versions dropped this, too). I don't think it's time for that yet, but Nintendo does like to keep cranking out new versions of their handhelds.