MK can't approach?
and there are a few matchups marth does better in, but why main the character with bad matchups? because he's more fun or you like him more, that kind of thing, but from a purely playing to win perspective, there's no point
His bad match ups are MK and Dedede.
Dedede is becoming a less seen character these days with the amount of high tier characters with advantages against him, and who doesnt have a bad match up with MK?
If you are going to throw out the play to win perspective then Ill take it to the extreme and say theres no reason not to use MK compared to any character.
Oh, and one of Meta Knights most publicised weaknesses is his approaching ability. His approaches, mind you, have LOLPRIORITY, but are mostly all telegraphed. He cant afford to pressure grounded opponents in the air like Marth can. His out of dash options have terrible range in comparison to the rest of his moveset (when you include his aerial mobility if he decides to aerial). MK does have a better dash to shield time frame though.
I think of MK as having some of the best punishment options in the game. But they are not too crash hot in the neutral situation.