Okay, so let's assume for a second the people who look at that tier list and are shocked don't actually know much about the character.
For one, his range wasn't nerfed on any move bar Forward Smash/Dtilt.
In fact in a lot of cases (noticeably his aerials), their range was BUFFED.
This listing shouldn't be surprising to anyone else, even if they think he deserves a bit higher (I doubt they think it's more than a lower-middle tier area).
But he's still terrible.
He definitely fits the bill for a low tier character in this game, but that may be a bit because we can look at how extremely terrible Lucina is and think "Marths just not that much better", although he is.
These are without customs either, which Marth does get some boons that may help his deficiencies (issues with hit confirms / kill confirms / grab game).
He was structurally guttered in every single way bar sword range. Every single way you could possibly think of.
Except his weight, and he got to keep his walk speed as no.1 in the game (although while all other movement options seem faster in this game, walking seems roughly similar speeds).
Grab range, damage, dodge timings, auto cancels, ending lag, aerial mobility, hitboxes, start up, etc etc
Marth got a similar treatment to what Falcon did for Brawl. All it would take is a few numbers to make him good again/solid, but Sakurai has decided to not have this happen.
The main factor/issue is that Marth does not have a single bread and butter tool left. He doesn't have any 'gel' to his kit, Fair, nair, dancing blade and down tilt are all bad moves in the grand scheme of things and he's left with nothing else compensated. MK was also heavily nerfed, but he retained two core tools in dash grab/dash attack (that are some of the best in the game, guaranteed) and this allows him to compete.
and 