Marth has no major advantages as a character, the matchup discussions are irrelevent because they largely factor in a player's ability to use the character and the player's ability to make judgements that affect the characters performance. Likewise Marth has no glaring disadvantages, hes light but fast, laggy attacks but relatively quick. As a character, Marth is reasonably well balanced. However, being a balance character puts him in MidTier range, since High Tier characters have something about them that inherently makes their character better than others, some strength that greatly outweighs weakness. MK has super duper speed, whorenado, godlike recovery and Snake half of snakes moves can kill, can set up traps with his mines, has explosives, projectile camping, and ftilt is a fast attack. DDD has CG -> Edqeguard, projectile with chance to kill, 5 jumps which boost his edgeguard, weight to avoid being killed, long reaching ftilt, and powerful attacks that make him one of if not the most effective camper in the game. Falco has his laser game, a cg for high damage at low percents, damaging attacks, and like marth has no glaringly exploitable weakness since autosweetspotting aids his recovery. GW's power:weight ratio is off the heezy, techchase dthrow, a good bair, good dtilt, and a plethora of kill moves, he even has a pseudo projectile. ROB has a reliable rocket recovery, 2 projectiles that do some good damage to work with, and quick tilts and a fast long reaching dsmash that make him a very effective camper. He may kill slower than Marth, but he can survive longer.
Marth has no projectile which leaves something crucial out of camping. He has no super speed like MK and his fastest attacks; Jab does not kill and DB is stale 99% of the time so it kills much later than it can. His kill moves are slow. He doesn't have the 5 jump edgeguard like MK, Kirby and DDD and due to his recovery he must be careful about going for gimps because a mistake can easily lead to being counter gimped and edgehogged.
In fact anything that Marth can do well, can be done to marth just as well. His bad Dair makes him succeptable to being juggled, his recovery can get him gimped, his laggy attacks can get him punished. And if he can gimp for a relatively low percent kill, his weight means he can also possibly die at quite a low percent also.
Everything about Marth carries some price. Thus I would argue Marth is possibly the most balanced character in brawl. Problem with being balanced is... half the cast is OP in some way or another.
Being a swiss army knife like Marth means the player must compensate for more weaknesses and exploit more advantages then other players, its a tall order. Marth is just not as easy to play as other characters that have something that screams 'abuse this aspect of me'. I would argue that the reason Marth is not playing well in tourneys is because playing marth well is a lot harder to do then playing MK well, and because of that Marth's performance suffers. And for the elite few who can say they play Marth very well, they still have to content with people who play MK, Snake, DDD, Falco and the other characters very well. Problem is those guys have a consistent game plan and a character that speaks for itself so its already a matchup in their favor in terms of getting settled in, then Marth has to deal with the fact that those matchups are widely considered in favor of the non-marth. On top of that for matchups where Marth goes even or slight advantage, he's still a harder character to play.
Add it all up and you get something along the lines of Good MK > Exceptional Marth.
Also, region does factor into placing greatly. M2K doesn't show up to as many WC events as he does East coast, in fact the population density on the east coast fosters more competition among a wider pool of players where WC smash not only has less players but also a lot of separation because we don't have closely linked major cities.
