Because you didn't realize until now that you want to defeat the likes of Ridley with Pokemon's equivalent of a rooster (and Torchic doesn't count!).
Tapu Koko is one of the legendary Pokemon introduced in Sun and Moon as the guardian of Melemele Island, which is the first island visited in the game and is home to the player. Sun and Moon are unique for Pokemon games as they are heavily reliant on a narrative that is pushed by legendaries. This mainly includes Cosmog's family, owned by Lillie, but also Tapu Koko, who takes on a guardian role of the player basically from the beginning of the game. Your player saves Nebby from a flock of Spearow, only to fall off a breaking bridge and into a raging river. Thankfully, Tapu Koko appears and rescues both Nebby and yourself, which is a big deal as Tapu Koko doesn't normally appear for any trainer to see. Further proving how special Tapu Koko is, the Pokemon gives your character the 'Sparkling Stone', which later becomes the Z-Ring, a central plot-important item that gives your Pokemon access to Z-moves. Later on, when Lusamine unleashes the Ultra Beasts into Alola, Tapu Koko appears to stop the attacks from Buzzwole/Pheromosa on Melemele. Finally, when the player defeats Professor Kukui at the Pokemon league, the player returns to Tapu Koko's shrine, where the Pokemon finally realizes that the player is now worthy enough, and Tapu Koko challenges it to a battle where it is eventually caught (this is different than any other legendary encounter in any Pokemon game as it's a part of the credits; you haven't even had a chance to save yet!)
In the anime, Tapu Koko plays a similar role towards Ash (I know people like bringing in the anime because Lycanroc so I'm bringing it up here). In the very first episode of the Alola arc, Tapu Koko gives Ash the Z-Ring and also teaches Pikachu Gigavolt Havoc (the Electric-type Z-move). In the following episode, Tapu Koko wants to battle Ash with his newly Z-powered Pikachu; the battle ends in a draw after Koko takes no damage and flies off. Later, it also watched Ash complete his first Island challenge against Totem Gumshoos. It later challenges Ash and Pikachu to battle. This one also ends in a draw after Ash and Pikachu fall off a cliff, only to be saved by Koko (sound familiar?). Once Nebby enters the anime, Tapu Koko and his fellow guardian deities take on a guardianship for the Cosmog, leading Ash to find Nebby and it watched its evolution into Cosmoem, and later help facilitates its evolution into Solgaleo, and allows both Solgaleo and Ash to perform Solgaleo's unique Z-move. Currently, it is working with the humans on exterminating the Ultra Beasts.
When compared to the Alola frontrunners for Smash in Decidueye, Lycanroc and Mimikyu, Tapu Koko finds a really nice balance in terms of relevance. It was one of the first Pokemon known about due to its brief appearance in the initial trailers, and as the player's guardian, it takes on a unique bond with the player much like a starter. Lycanroc does have more anime importance, as does Mimikyu (debatable), but neither match Koko's role in the games.
TL;DR: Story significant Legendary Pokemon in both video games and anime, which doesn't really exist in any other form.
As an Electric/Fairy type Legendary, Tapu Koko has a lot of unique abilities. This includes its in-game unique ability to summon electricity from nowhere using its Electric Surge ability (the Electric Terrain field move). This basically has the effect of buffing its Electric-type moves. To go along with this, it carries the usual assortment of Electric-type moves in Thunder/shock/bolt, Discharge and Electro Ball. Being based on a bird, it also carries a few Flying-type moves such as Sky Attack, Brave Bird and Acrobatics. Finally, as a Guardian Pokemon, it carries the unique move Nature's Madness, which always does half-damage to the foe. This can be upgraded using the Tapunium-Z item into the Z-move 'Guardian of Alola', which does 75% health damage to the foe. In terms of stats, Tapu Koko is fast, favoring its high speed stat, with a high attack stat to compliment it. It's special attack is also pretty good, but it suffers from low defense.
Now I know 'fast Electric-type' sounds a lot like Pikachu, and I think that's why Tapu Koko is a virtual unknown in the Smash fanbase. However, Tapu Koko differs in its fighting-style. This Pokemon loves to fight! The PokeDex mentions that it becomes 'malevolent' when its 'hair-trigger anger' is unleashed, suggesting that it's a very reckless Pokemon. In Smash, this could be viewed as a character that simply gets joy out of beating the tar out of Ridley or Villager. When compared with Pikachu, who has an evasive fighting-style owed to its small frame (using its quick speed and Quick Attack to dart in and out), Tapu Koko simply wants to get in, close the gap and unleash the assault, truly not caring at all if it survives (it's a Legendary Pokemon so it's basically immortal anyways!). Such a reckless fighter has more in common with Captain Falcon than a wimpy mouse!
There are a few ways this works into a Smash moveset. Using its Electric Surge ability to create the Electric Terrain, this could be a charging buff move (ala P:M Lucas) that buffs Electric moves damage for a duration. This could be a field-effecting move as well, but as Inkling seems to have field control as its main attribute, we'll leave it alone. As Tapu Koko's unique ability, this seems like the most unique route to go as a fighter. It's got movement based moves in the form of Acrobatics, Wild Charge and Sky Attack to close gaps to get into the opponent's face, from there being in a comfortable position to unleash Electric hell on the opponent. Their fault for challenging a legendary I guess!
You could also go for a more reckless set centered around debuffs. Central to this would be the move Electro Ball, a move which does more damage if the target is slower. Obviously this can't be translated into Smash as is or it'd be too useless in certain matchups and absolutely ruthless in others. However, what would work is Koko using moves like Mean Look or Thunder Wave to inflict a speed-debuff on the opponent, and then using something like Electric Terrain to buff its own speed and increase the damage of Electro Ball. I like the first set more for its simplicity, but feel like the second set creates a uniquely frustrating scenario that tries to mimic the difficulty of catching a Legendary Pokemon. Either way, the Final Smash has to be Guardian of Alola - it simply makes too much sense for it not to be.
In terms of stats, Koko would have excellent speed stats, rivaling Greninja in terms of ground speed, falling speed and air speed (this is one of the fastest Pokemon ever canonically!). It's also 6 feet tall, so it would be tall, probably similar to Charizard. And while Sakurai doesn't always take canonical weights into account (hi Mewtwo!), Koko's reckless nature would seem to fit a lighter weight fighter, probably around Zelda's weight. This is very much a glass cannon character, something which a lot of characters try to be, but don't always make canonical sense, unlike Tapu Koko.
So yeah, here's another Pokemon to add to the Alola group, one that seems to make a lot of sense due to its importance in lore that allows it to stand out from the crowd despite face-level similarities to already existing fighters.
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