DKII on 21 November 2008
Lower profit margins on the games themselves (they require actual work to convert/emulate). Other stuff already mentioned (fills up space quicker, can't use the memory pak, etc.) Also don't neglect the factor that they want to milk the earlier games before releasing the sequels. An example of this is F-Zero, with the N64 version out the SNES version is barely selling at all.
