What makes you say that? From what it looks like, 4v4 is fundamentally different like being limited to certain stages only.
It's easier to solve problems than to add new features.
For example:
Let's say items in Brawl would have had a good chance at making Brawl a
fundamentally better game. That items would likely have been much better for the community and the competitive lifecycle if they had been implemented correctly.
Adding items themselves is a huuuuuuuge undertaking and you have no real way of knowing where to start!
Let's say the perfect item list is green turtle shells, food, and sticky bombs.
If we start with
all items we can easily trim down the item list to remove things like invincibility stars, bob-ombs, etc., without much issue. Since green turtle shells, food, and sticky bombs are the perfect item list there would be no reason to remove them so they'd end up being what we went down to in the end.
If we on the other hand decided "I know best" and declare "Our item list will only contain Green Turtle Shells and the Star Wand", we'd get it wrong off the bat. We'd have absolutely no way of knowing to add food or sticky bombs -- if we're observant we might find removing the star wand is good, maybe. But when it got down to green turtle shells someone would say "this is good enough" and there would be complaints as to the changes of food and sticky bombs because people hate change.
If we start with 4v4 and find "4v4 is fun and awesome", then we find no reason to play 3v3 -- no incentive.
If we start with 3v3 and find "3v3 is fun and awesome", then we find no reason to play 4v4 -- no incentive.
But if we start with 4v4 and find that 4v4 is chaotic and not fun, we can
scale down to 3v3. We have incentive to do so.
If we start with 3v3 and find 3v3 is chaotic and not fun,
there is no incentive to scale up to 4v4.
It's the same with all things. If you play only on FD, suddenly Yoshi's Island seems like a terrible and janky stage. Scaling up involves change and adding new features. Scaling down involves removing unwanted features.