Why? Because Brawl's cutscenes were uploaded online.
In his bi-weekly column in Weekly Famitsu, Super Smash Bros. creator Masahiro Sakurai revealed some new details about the upcoming game. One major detail he covered was the exclusion of a story mode and CG cutscenes and his reasons behind it.
In the previous game, Super Smash Bros. Brawl, the development team took advantage of the rare occasion of having multiple different series' characters all appear in the same game, so they painstakingly created a special story called the Subspace Emissary, complete with cutscenes for players to enjoy. It was a project that obviously took a lot of thought, time, and energy. And then, the obvious happened...
"Unfortunately, the movie scenes we worked hard to create were uploaded onto the internet." Sakurai lamented. "You can only truly wow a player the first time he sees [a cutscene]. I felt if players saw the cutscenes outside of the game, they would no longer serve as rewards for playing the game, so I've decided against having them."
This is unfortunate. The cutscenes during Subspace Emissary were kind of awesome.