That was the reason, Sakurai got mad over the brawl cutscenes being uploaded into youtube since he felt they should be a reward for playing the game proper instead of simply being a click-n-play thing on the internet, and then he cut the game mode entirely since he saw that we would do it again if he did add a story mode again.

Well, I heard one rumour that Sakurai didn't develop an adventure mode because he didn't like how Brawl's turned out. And, that's understandable – it had some major flaws.
Then there's another rumour that Sakurai didn't develop an adventure mode because he didn't want its cutscenes to be recorded and revealed online. Which I hope is false, because that reason is completely nonsensical.