Why some Games are NOT available:
FFVII is not listed because it uses 2.74GBs of Memory (It used 4 Playstation 1 Discs) and not very many consumers have that much memory. Not to mention how long you'd have to wait before it is downloaded. Not only if Nintendo would have done that, then they'd be able to sell Gamecube Games Via Wii Shop Channel (2.4GBs = 1 Gamecube game Disc).
Mario Party 1,2, & 3 are owned by a 2nd Party Company (Hudson) so they control if it's available.
Super Mario Allstars for SNES was a Deluxe which included Super Mario Bros., Super Mario Bros. 2, Super Mario Bros. 3, and Super Mario Bros: The Lost Levels. It's not available because they want you to pay for them separetly, and to have the original versions of these games. Otherwise they loose money.
Donkey Kong 64 may be owned by "Rare", I've never played it but they've made most DK Games. Diddy Kong Racing isn't available because of one Racing Character is owned by "Rare".
Nintendo Games Owned by Nintendo get place up there by a board that chooses what becomes available.
All the one I mentioned:
Doom, Doom II, Doom 64; iD owns the rights.
Tony Hawk Pro Skater 1 & 2 (THPS1&2); Are owned by "Neversoft"