It's funny how when Luffy shows growth in strength, threads like this show up accusing Oda of giving Luffy special treatment.
Anyway, Luffy is the MC, we are following his story and mostly his fights, it only makes sense that a lot of things we see are through him.
Other characters have also shown haki bloom, it's just that the ones who have the most focus are the strawhats, take Zoro for example:
Zoro - In Alabasta Zoro awaken the breath of all things. Against Kaido, who was his first tough opponent in the new world, he awakened conqueror's haki and advanced conqueror's all at once, which is the biggest haki bloom to date.
As for Sanji, I'm sure if Oda didn't go the Raidsuit route he would have given Sanji future sight or some other advanced observation haki in Wano.
Guys like Law went from struggling against Vergo to fighting Kaido and Big Mom, clearly they grew, we just didn't see it.