As mentioned above, marth has several weaknesses - his reliance on edgegaurding to kill, lack of projectile, and his speed.
Marth's primary killers are his dair and fsmash. Both these moves have to be setuped in order to work (eg. out of a combo or cg). However, marth's combos are reliant on %.If a marth's opponent reaches a high percent, marth can no longer combo them and finish them. He has to rely on edgeguarding to kill them at high percents. All other top charactors in smash don't have this problem. Fox has usmash, uair, and his homosexual shine spike. Falco has his dair, bair, and his dsmash. Sheik has fair, bair and her dsmash. Marth's ko moves have to setuped.
Marth has no projectile. He is forced to approach his opponent. In this way, any charactor that has a solid projectile holds a degree of control over marth. Fox, Falco, and sheik all have projectiles, so they can projectile spam if they dont like the situation. Marth cant do these, he is almost always forced to approach.
All the other top charactors are quicker then marth, combined with their projectiles, they can projectile spam against marth very effectively. Additionally, their high speed makes it difficult for marth to space his attacks effectively. Once a charactor gets insides marth's range,hes screwed. Marth's range his largest asset, and quick charactors can negate this.
Basically, any charactor that either possesses good range, or a lot of speed, or a solid projectile does good versus marth. They are quite a few charactors that possess these attributes(or some of them), such as sheik, falco, fox, ganon, link, just to name a few.
Space animals can combo nearly any charactor in smash very well at nearly any percent. They have a good projectile and are very fast so they can manipulate their opponent, and they have no real problems killing their opponents at any percent (stupid gay fox with his gay shine). Because of their solid offense, and their solid defense they perform well against all charactors in melee, and have to no real bad matchups. Their only real weakness is their vulnerabilty to grab combos.
Same with sheik. All her attacks lead to her fair, she has an excellent projectile, and has a lot of speed. Again, she has no bad matchups (except for IC's)
Marth possesses weaknesses that the other top charactors dont have, and that is why he belongs where he is on the tier list IMO.