I guess it's fun to speculate why Spelunky XBLA has not been released:
1) It's too much fun : Maybe Derek and his friends/fellow creators are having so much fun with the dev build, they can't get the final out of the door. FTL's Dungeon Master nearly didn't get made because the dev team kept playing Oids. Or was that Carrier Command nearly turning to vaporware? I forget. Point is, could happen.
2) Feature creep : The XBLA version has more features than the PC version. Derek+co are experimenting with more and more features, and it could always have more stuff.
3) Bugs : PC original didn't have any game-breaking bugs AFAIK, but the more feature-rich XBLA version might have, particular due to feature creep.
4) Multiplayer & netcode : Other programmers might be content releasing games with poor netcode, but not Derek. This part might be REALLY hard, more than any non-programmer could know.
As for myself, I would be perfectly happy to buy Spelunky NOW, just with the vsync and HD graphics, even if it had no multiplayer and no features beyond the PC version. The screen-tearing in the Gamemaker engine is making my eyes swim.
But Derek doesn't want to release it in that form, and that's that.
Duke Nukem Forever was considered vaporware by many, and that eventually came out too. Even if it takes as long as DNF, Spelunky would end up a better game. Even Vulture's Eye (nethack) is still interesting to play today, and I think the Spelunky concept would have similar lasting power -- which the Duke Nukem concept does not.