Possible money constraints. As I said, Nintendo hasn't been doing the best, and while I know pretty much zero about economy and all that, Nintendo just might not make any money by dealing with SEGA, in fact, they might even
lose money. I'm not saying the Dreamcast library sucks (Soul Reaver, Soul Calibur, Sonics, Shenmue, Ecco the Dolphin, and those are just the AAA titles), but the possibilities are still there
I'm all for the Dreamcast coming to the Wii U -- the file size for an ol' DC game is roughly around 600MB which isn't all that much (GameCube games, which, as I recall ARE going to the VC, are around 1GB on average). Just, money. idk.