Why does the Smash Bros. announcer say “Rainfall!” in the Wii U character select screen

Supposedly there are rewards, but Nintendo hasn't detailed what exactly they are.

Simply having points isn't good enough, your character also has to win. You can see which character is currently winning by accessing Conquest Mode on the Online menu.

You earn points in this mode by playing as one of the characters in a match where both characters are present. For the current Conquest, that's Mario vs. Megaman.

Last I checked, Mario was in the lead.

The requirements to unlock each stage can be found in this blog post:

• Kongo Jungle 64: Clear the "The Original Heavy Weights" event in Event Match (single player).

• Smashville: Clear the "Playing Tricks" event in Event Match (single player).

• Pokemon Stadium 2: Clear the "Lightning Strikes" event in Event Match (single player).

• PAC-LAND: Play all the maps in Smash Tour.

• Flat Zone X: Play alone as Mr. Game & Watch and destroy 100 blocks in a single game of Trophy Rush.

• Duck Hunt: Get one or more KOs in a single Solo Cruel Smash.

I have confirmed all of these methods and that these are all of the unlockable stages.