Brawl as a national competitive game in North America (that will be my focus on this post BTW) died largely after APEX 2014. I've heard of more Smash 64 tournament streams than Brawl ones since APEX 2014. I also can't think of 1 Brawl tournament that got over 5k simultaneous viewers since APEX 2014. In the meantime, Melee and Project M at Xanadu beat that number every week, and dozens of other tournaments have done the same.
Brawls death as a national level game happened because:
1. The lack of Brawl streaming (from major streamers, consistently weekly streams, monthly streams, and viewership).
2. The scene kept losing older players and not replacing them with newer players.
3. No Brawl nationals being planned after APEX 2014, with no organized movement or schedule to push this.
4. The money shift from Brawl to Project M and especially Melee and now Smash 4.
5. Lack of a demand and hype for Brawl (it is why sponsors and major events avoid it; it's perceived as "boring" and "old")
6. No plans to keep Brawl's scene together once Brawl's newness wore out (with the releases of Project M 3.0 & Smash 4).
I don't care what game we're talking about, that would kill any game's competitive scene at a national level.
Smash games promote each other, and the more we have that are national level, the better attendance, viewership, and interest tends to be across the board. A lot of the success of Project M and Melee nowadays came from Brawl's success and the scene's size, specifically Brawl's growth from 2010-2013. Fortunately for the rest of the Smash scene, that impact won't be too big, due to Melee just becoming HUGE, Project M being a consistent national level draw, and Smash 4 inevitably being a hit.
Still, Brawl's national level disappearance still hurts the community. With that success and size gone and in the past, Smash 4 won't have that opportunity and crowd to build off of, even with the new/recent Melee and Project M audiences.
As a consolation, APEX is always a reliable showing for Brawl, just like it is for 64. If some people who still love Brawl are smart, they can use APEX 2015 as a relaunching point for that game and create a schedule for local weeklies, monthlies, bimonthlies, and even national events in hopes of reigniting the Brawl scene.
We all know that would be what's best for the Smash scene as a whole. Without Brawl as a national game, nationally all smash games and most local scenes lose a potential market and audience. Let's hope that is curbed, hopefully soon.