Because it's not!
Kirby players don't like to admit that the character's bad, but like... what are Kirby's WINNING matchups? I think he might, MAYBE, have 5. I used to list Mewtwo as a winning matchup, but that was before Mewtwo got great buffs, and even then it might have not been a winning MU.
Kirby players like to say they beat Fox. This is true, if Fox freely runs into your Uptilts. But he won't! Because his neutral is far better! Because he's fast!
Kirby vs ZSS is not even! Crouch is good against a lot of her moves, but you're not threatening anybody when you crouch! And she can still hit you with Dsmash, Dtilt, SideB, (well spaced) Nair, etc! And she kills you insanely quickly! Upair chain -> UpB kills Kirby with the "gimmick" hitboxes, the ones that ceiling KO you before the final hit even connects, more often than you'd believe!
Kirby has some cool things vs Sheik, but it's not in his favor! I could argue for it MAYBE being even if Kirby has her Needles, but that requires actually getting them. And if Kirby's increased weight means Dthrow->Upair is no longer a guaranteed kill (GUARANTEED, mind you, it wasn't an airdodge 50/50) then that might help. But aside from that, we already know why Sheik is a top tier, and Kirby isn't.
When I tried to list Kirby's winning/even/losing matchups many months ago (like, 5 months) I had ~6 characters as winning matchups. Why don't I just go through the entire tier list and show why I think he's bad.
Top
S:
Kirby definitely loses to these dudes. All of them become surprisingly "more manageable" if he manages to Copy them, but again, Inhale is not a move you want to rely on in a high level tournament scenario. Despite the buffs it's gotten, it's still free damage if you make the wrong read, and Kirby's light weight does not agree with taking free damage.
High
A:
Kirby loses to these. I can see arguments for Fox/Pikachu being maybe even, but the rest are nightmares. Their Copy powers also do little to help!
B:
I'm not sure about Villager, but I can't think of any distinct advantages Kirby has in the MU, besides maybe stealing Pocket. Cloud is the usual "wins onstage, loses offstage" but Cloud wins onstage pretty hard. Ness is basically the same boat as Cloud, as far as Kirby is concerned.
Middle
C:
Pretty close to even against Falcon. Definitely a loss against Yoshi. Kirby does not have an advantaged state in this matchup.
D:
Luigi wins, at least a little. I'm pretty sure Dthrow->Cyclone is still a true KO combo on Kirby, at specific percentages. Pits I'm not sure, I imagined they were close to even. Peach seems pretty even, and depending on how effective her technical high damage combos/infinites are, it might even be in Kirby's favor! ROB is evenish maybe. His attacks outspeeds/outranges us on the ground and air, so lol. He's combo food though. Wario feels even, until he Wafts. Toon Link, not sure. Lucario is a terrible matchup for Kirby, since he has no reliable way of ending stocks before Lucario becomes God.
E:
Olimar is a nightmare matchup, until Kirby gets the power, then it's Kirby's favor. So.. still a bad matchup, because it's extremely difficult to land Inhale on him. Greninja wins a little, the mobility is that important. Ike is a pretty even MU, I feel. Not sure how Ike's throw KO confirms work on Kirby though. DK beats Kirby for sure. Mega Man is... super annoying. Probably MM's favor, as per usual. Pacman seems evenish maybe. Kirby has to get close to Bowser to fight him, and his speed is slow. Just imagine how that matchup goes! Robin's projectiles are easy to get around, and his recovery is easy to edgeguard, but dthrow->levin Upair WRECKS Kirby, so the matchup's pretty even.
Low
F:
Not sure about Roy. Bowser Jr seems pretty even. G&W seems pretty even. Lucas seems pretty even. Mewtwo seems pretty even. Falco is annoying because all his moves come out faster and are disjointed, but also seems pretty even. WFT struggles against Kirby's crouch and combos and edgeguarding, and Kirby does very well with her power. Might be a winning matchup.
G:
Shulk is tough unless you get his power. Evenish though. Marth has always been a personal nightmare for me, and he just got major buffs, so I would say it's a very losing matchup. Link is pretty even, we can't land against him because Grab, we can gimp him and win in super-close CQC, but everywhere else we lose. Duck Hunt plays keepaway very well against Kirby. Little Mac ... maybe you can win if you just camp the ledge and wait? Dr Mario is pretty close to even, but his speed/power ratio is no joke!
Bottom
H:
Dedede loses to Kirby in most regards, but he takes so incredibly long to kill that he has a ton of opporunities to make up any deficit. Lucina is basically Marth.
I:
Miis aren't real characters, Charizard seems evenish, Palutena seems evenish.
J:
Kirby uses Charge Shot better than Samus, and wins in CQC and combos her well, might be in his favor. Miis don't exist. Ganondorf is combo food, but amusingly, his Dtilt beats anything Kirby has on the ground, and his Upair beats anything he has in the air. It's a battle of extremes, I'd put it close to even. Jigglypuff wins in air-to-air, Kirby wins ground-to-ground, but the pace of the matchup feels like it's in Jigglypuff's favor because of air camping and stuff. Zelda idk. Playing really defensively against her is probably Kirby's victory.
Kirby's too "fair" of a character. He doesn't have any crazy stuff that makes it seem like "yes, Kirby definitely wins this matchup because (insert overwhelming trait here)!" The simple truth is, he has to suffer through his terrible neutral, rack up damage until usually 100% or higher, then land a smash, or knock them offstage and get a successful Bair/Dair, or get them even higher and then Bair, or Upthrow on a platform. He doesn't have any safe/early KO confirms, so any time he has an advantage in a matchup, the opponent will have multiple opportunities to even it back up.
That taken into account, I feel his matchups look something like this:
Possibly winning matchups (that I'm sure mains of these characters can argue against):
Pretty even matchups:
Losin':
Feel free to ask me to elaborate on any of these if you're interested. Note that there are a few I don't know about. One example is listing Ike as an even matchup, but maybe Ike DOES have a guaranteed throw kill combo that I don't know about, but any time I've played an Ike, it's seemed that Upthrow sends me too far to followup with an aerial at kill range. Still, taking Kirby's strengths and weaknesses into account, I doubt there are any more "winning" matchups, and I could see some of the winning/even ones being worse than listed, etc.
Yes, I am pessimistic about this character. I was plenty optimistic about him for about the first 6 months of the game's life, but it's just obvious that he doesn't have what it takes to compete. He's gotten a slew of buffs, but they're targeting the wrong things. Yay 2 more weight! Yay ~2 more KBG on 4 moves!