But wait I really wanna know what made Little Mac so special that he basically shows up in Brawl and then is playable next game... Why not just make him playable in brawl? Eh whatever.
I can explain this one.
At the time of Brawl, Little Mac was just another retro character that hadn't been used in over a decade. Granted, he was a popular one, as Punch Out!! was a huge franchise in the west, but just ultimately one of many. Sakurai made him an AT in Brawl to celebrate that popularity.
But after Brawl, Punch Out!! got a very successful reboot, selling several million copies. The Wii game ultimately did two things for Little Mac.
1. It made him a relevant and marketable character.
2. It made his popularity and the popularity of the Punch Out!! franchise skyrocket.
Little Mac became such a popular request during Pre-Smash 4 that he was considered one of the top ten popular requests. Yes, back then, he was among the popular kids club like Mega Man, Ridley, K. Rool, Mewtwo, Palutena and Isaac.
The stars aligned perfectly for his upgrade. And the other two AT promotions had similar cases.
Isabelle was a popular character during Smash 4's development, but after Smash 4, her popularity surged. She became one of the most popular modern Nintendo characters, constantly appearing in marketing, usurping the role of Animal Crossing's ambassador and even nabbed a spot in Mario Kart. Isabelle was too big NOT to be included at the time of Ultimate's development. She and Inkling were the faces of modern Nintendo.
Dark Samus got extremely lucky. Sakurai needed an echo and Dark Samus did well in the ballot. Considering she hasn't appeared since 2007 and is canonically dead and won't appear again, she's potentially the luckiest character on the roster. She just filled that echo spot perfectly.
Characters who were previously ATs either had massive developments after their initial inclusion or fit the role they needed perfectly.