He's just not...that good?
Pit doesn't have many flaws, he's just not that good.
Marth's like that, he doesn't have the strengths other characters have (he has range, but range alone doesn't put you on the level of an infinite against the entire cast, or bananas, or lasers + chaingrab, etc.)
His specific weaknesses? He has a really hard time dealing with cross-ups, and he has no fast moves besides up-b (whereas other characters have their jab). He has a blind spot below him in the air and a bad air-dodge making him quite susceptible to juggles, his fair often gets really staled, and his throws do no damage.
His recovery is linear and he suffers from RCO lag (which like no other top tiers besides Falco suffer from). He kills later than most characters (without tippers)
His matchup with MK is one of the worst compared to those with similar tier positions, his matchup against ICs is lackluster.
Marth's always rated very highly and people say he has a great matchup spread, but it's never really demonstrated (to the level of other top tiers at least, Marth's still a good character).
If you want top tier strengths Marth has, then I can give you some. His large and almost lagless aerials gives him an insanely good pressure game (he can limit options extremely well, he is like MK in that way) and an extremely effective zoning game. With the pressure he can put he can string very well and keep you in a disadvantageous position (like on corners and in the air) for a very long time where he can rack up damage very fast. He has arguably the best ledge trapping and definitely is top 3 or top 4 when it comes to juggling. He can punish almost anything you commit with, including OoS (yes, even MK can't safely hit Marth's shield most of the time).
His recovery is average. But it doesn't suck. Only a few characters have tools to reliably gimp Marth (like MK, DDD and ROB). Other characters will rarely gimp him if the Marth knows how to recover. RCO lag sucks sure. It sometimes can hurt Marth, but only when on the ledge when above 100%. Marth has some problems on the ledge on those %s, because he loses his amazing ledge jump invincible aerials and options like a great ledge climb (his above 100% ledge climb sucks a lot compared to his below 100% version, same with his ledge jump).
Marth doesn't lack at close range. Marth's jab and DB are both 4 frames and if it counts as a close range option, shield (because of his really good OoS options). I've never had problems at close range. Marth happens to have good options for getting out of close range as well.
The cross-up thing isn't something I'd call a weakness. Marth can deal with it really well with the help of something called reaction. With great spacing he can even retreat so much so that the opponent be unable to roll behind him (even if the opponent can roll behind him, most of the time you can still react and punish with DB or Up b unless it's like MK's forward roll). Most of his moves not hitting both sides is only a weakness in doubles, not singles. It's true about being susceptible to juggling, having bad options (his Dair is pretty bad at protecting from above) to protect himself when the opponent is directly above does suck but he's not terrible at dealing with juggling. Most of the time if he gets hit in the air he'll always get back to the ground after taking 1 or 2 hits. His air dodge isn't terrible despite being the 2nd worst frame data wise. Marth's air mobility and good fast fall makes his air dodge usage surprisingly far from the 2nd worst in the game. It's true that Marth kills later than most characters without tippers, that's why Marths have godlike spacing to almost completely remove that weakness. With really good spacing you can tipper with every aerial 65% of the time.
Sure his throws themselves do little very damage, sure, but they don't suck because they put the opponent in a such disadvanategeous position that he can follow-up with it most of the time. And at around 0% he gets true combos out of Fthrow like tipper Fsmash and aerials.
Fair getting stale a weakness? Don't make me laugh. Fair can kill, but he has other reliable kill moves so he doesn't need it to KO. And staling it only makes it a stronger combo machine. Also, if Marth needs to refresh something, then DB is GODLIKE at it because it counts as 4 hits when staling DB. Marth has actually arguably one the least problems in the game when it comes to staling.
And finally, Marth doesn't lose to ICs. It's an even MU. Marth loses to MK, but it's not a bad MU. Mikeneko at worst goes even with every top MK in Japan. Personally I think the MU is relatively close to even, but it's still a -1 (only harder than like Diddy's, Fox's and Falco's).
