Oh man why do I get the feeling this is banned.
Why can't fun stages be legal on Tournaments :/
Alright, this has been a long time coming.
Ahem.
THE BIG BAD LIST OF WHAT BANS A STAGE AND WHY
1. Walk-offs
Walk-off stages were banned, in part because of chaingrabs, but also because they heavily promote camping at a point in the stage where b-throw kills at 0%, which isn't fun to play or watch. They were also banned because they completely remove the edge game and off-stage game, which can completely cripple some characters.
Examples include:
Yoshi's Island (M+B)
Mushroom Kingdom 1 (M)
Mushroom Kingdom 2 (M)
Bridge of Eldin (B)
Distant Planet (4+B)
Gaur Plane (4)
Gerudo Valley (4)
2. Extreme Character Preference
These stages are those that provide an unfair amount of advantage for one or more characters, to the point that they have almost free wins on those stages.
Examples include:
Mute City (M, Peach and Jigglypuff)
Rainbow Cruise (B, Meta Knight)
Brinstar (B, Meta Knight)
3. Gaps
If a stage has an area of open space breaking up segments of the stage, it provides free wins to characters with spikes or meteors, and promotes camping by those characters on the other side of the gaps.
Examples include:
Jungle Japes (M)
Green Greens (M+B)
Norfair (B)
Reset Bomb Forest (4)
Find Mii (4)
4. Walls
Walls promote infinite combos and inescapable chaingrabs.
Examples include:
Corneria (M+B)
Onett (M+B)
Venom (M)
Delfino Plaza (B)
Shadow Moses Island (B)
5. Match-Influencing Scrolling
These stages are scrolling stages that, by virtue of obstacles, speed, or both, significantly influence the pace of a match without any player input.
Examples Include:
Icicle Mountain (M)
Poke Floats (M)
Rainbow Cruise (M)
Mushroom-y Kingdom (B+4)
Rumble Falls (B)
Summit (B)
3-D World (4)
6. Overly Powerful and/or Non-Indicated Hazards
These are stages which place a large amount of power in non-player influenced events, which may break combos, or worse, instantly kill.
Examples include:
Peach's Castle (M)
Corneria (M)
Pictochat (B+4)
WarioWare Inc. (B+4)
Pokemon Stadium 2* (B)
Hanenbow* (B)
Golden Planes (4)
*Not banned in all regions
7. Circle-Camping
These are stages (usually very large) that allow a player to go from one part of the stage to another in a path that can completely circumvent contact with an opponent, which heavily promotes stalling.
Examples include:
Hyrule Temple (M+B)
Yoshi's Island (Past Stage) (M)
Kongo Jungle (Past Stage) (M)
75m (B)
Luigi's Mansion (B)
New Pork City (B)
8. Game-Changing Nature
These are stages that completely change the nature of the game, the objectives of the game, the dynamic of characters, and so on.
Examples include:
Mario Bros. (B)
Balloon Fight (4)
Green Hill Zone (B+4)
9. Caves of Life
These are areas in a stage where walls and/or ceilings create areas where it is difficult or impossible to take a stock, promoting camping in said areas.
Examples include:
Brinstar Depths (M)
Great Bay (M)
Spear Pillar (B)
Arena Ferox (4)
Please note that it is very possible for a stage to be banned for reasons not listed here; this is intended as a general guideline for stages. If a stage has any of these things, it's probably not getting through. Also, it is very possible for a stage to be banned for multiple reasons.
TL;DR
This is why 'fun stages' aren't allowed in tournaments.