or it's because they have been determined as low tier and thus no-one uses them because they don't want to use a character that others have determined as worse when compared to other characters. (im not saying that their assessment is not true, im just saying that this is how it is in a lot of cases)
imo the biggest differance between a low tier and a character of a higher tier is that while overall those characters may have near equal abilities to succeed in general, low tier characters have 1 or more glaringly abusable disadvantages whereas those above them may have either pure neutrality or a glaringly abusable advantage.
for my example in my comparison im going to use link, peach, and metaknight.
lets look at link as an overall character. his ground game is overall not that bad. he can play a very safe on stage projectile game, has sword priority, decent range, good zoning and spacing capabilities with zair and bombs, and has kill power. due to his fall speed and weight he can DI and live until snake like percents. he has a good zair, bombs, and a plethora of ATs. however he has the glaring disadvantage of having a poor recovery and combined with his fall speed allows him to be gimped at an above average rate. this one factor deters many from using him as they are not willing to take the time to learn how to avoid this disadvantage and use what he does have going for him.
now we look at peach. she has an overall balanced game in all aspects whether it is zoning, recovery, killing, ATs, general priority and spacing. however while she does not have any glaring disadvantages she also does not have any glaring advantages in any single area. thus people are more willing to use her that is why you actually do see peach in competitive gameplay.
now we look at metaknight. not only is he balanced in listed categories as a character, he has glaring advantages practically all of them. his range, recovery, priority, zoning, and spacing are practically unrivaled in this game. thus he gets the most attention as a character overall and thus he is the most popular character in competitive brawl.
that is my take on the whole thing anyways.
