The ironic thing about Porky is that the only reason he's wanted for smash is as a result of Smash itself. If Ness and Lucas had never been added, Earthbound would be dead, forgotten, and irrelevant. And, if it somehow had any support, it would be for one of the protagonists, not Porky. Objectively, he's about as needed as the fish from Baloon Fight. There's also the spirit thing: If they had just added a Proky spirit, there wouldn't be nearly as much demand for him.
I always feel people overexaggerate Porky's importance. The reason he was added as a boss in Brawl was because he was the main antagonist of Mother 3, which was relevant at the time of Brawl's development and later release, just like Rayquaza.
He just happened to be also a main driving force in Earthbound as well, and let's face it, Porky doesn't hold a candle compared to Dracula, Ganon, or even Marx (because you bet your ass Sakurai will always show an opportunity to shove in Kirby content from games he made) in terms of importance and general recognition. Weirder omissions have happened in the past.
A lot of people thought that Isaac would be DLC in Smash 4 due to his absence as an AT, the only content we got was the Brawl Remix and a direct Rip from DD. I mean if Tekken could come with a random Heihachi Mii and then later with a full on pack, then the same could have happened with Golden Sun right?
Oh and i also want to say that Diddy Kong, DK's Buddy, who was the MAIN protagonist of 2 DK games, was nowhere to be found in Melee outside of maybe a passing mention. THAT is a far more glaring omission that Porky will ever be.