They don't have the advantage of being faster, speed is an innate character stat that is balanced against all of their other statistics and those of the other characters. It's more complex than this, but this is the general idea: if a character is slow, they are also usually heavy, if they are heavy, than most of their attacks are very powerful, if a character is fast, their usually light, if their light, than most of their attacks are weaker. There are so much more variables that add into it such as range, priority, knockback, lag time, duration, and so on, but character traits are balanced.
L-canceling and wavedashing are *physics characteristics* that affect the entire cast of characters in a set way. L-cancelling was a characteristic that interacted with a characters attack speed and landing lag statistics, wavedashing interaced with a characters jump start up time statistic, air dodge statistic, and friction statistic. Characters with higher statistics in those areas gain a lot more from those physics characteristics than other characters with NO COMPENSATION to the characters that didn't gain as much from them. This would be like giving a slow character nothing but weak attacks, their not getting compensated in any way for being slow.
Specifically, characters like falco could reach zero lag time with l-cancelling paired with some of their faster moves. They have those moves for alot of reasons, their light, they're fast fallers, they don't do alot of damage, etc. Thus, they are ALREADY compensated for being so by having moves that are already so fast. The problem comes up with the zero lag time state of certain moves, this is overcompensation for the fast character, because the slow characters cannot gain any zero lag time moves in order to be on equal footing with the fast characters. The obvious rebuttle to this is that they don't need any zero lag moves because they've been compensated by being very strong, thus, they ARE on equal footing as far as balance goes. However, zero lag moves break this idea, because zero lag moves have special implications such as the potential of semi-infinite combos, and un-punishible attacks. Slow characters have none of those, which is why L-cancelling is unbalanced.
This issue isn't as simple as people make it, and if you're someone who believes that a game must favor speed to be technical, then you don't have balance in mind, and all characters WOULD have to be the same speed in order to be balanced.
