generally because the protagonist calls on the strength of their bonds in order to accomplish their tasks. apologies if i made it sound like something that was confirmed -- this is just my own interpretation, since you use the "strength of bonds" to well. do everything in persona. makoto/kotone needs it to seal nyx. yu needs it to defeat izanami. ren needs it to defeat yaldabaoth. you hear their voices as you do that, or see them physically in p4

stuff related to the rank 10s shows up in the adaptations too ^_^ the p3 movies, manga, p4 manga and anime, and p5 anime (the manga's not done) all depict the protagonist calling on their bonds. often you can see them experiencing story beats in links across the adaptations as well. though their featuring in adaptations not as strong as an argument of "well it's how you beat the game so that's why i think this"

as for the p4/p5 cast's spinoff personas, really i just think it's a matter of the original personas being more marketable and recognizable. and atlus being able to get away with it because they're evolved through social links (which not the majority of players are not going to 100% --) unlike persona 3 where finishing the game necessitates you to have the persona evolutions. sort of like a "well the player probably won't finish all of the links, and the original personas are way more recognizable anyway, so why shouldn't we just use the ogs"

i forget if it was ever mentioned in an interview before, but i feel like i remember an interview with some project manager at atlus explaining the decision. don't quote me on that (not that i remember what was said anyway), i just i have talked about it a few times before in reply chains and feel like it came up at some point. personally though i just have always viewed the rank 10s as canonical because it doesn't make sense to me to be arbitrary in choosing which ones are and are not maxed. my view personally is that they're all maxed throughout the story, and the cultivation of these links is what gives the protag the power to accomplish their end feats

plus if you want a narrative reason then like idk. the 4 and 5 gangs splitting up at the end of their arc decayed their bond enough to put them back to their original personas, or it was only through the objectives of the narrative that they were able to access this secondary evolution. i feel like you can make a narrative reason for it if you tried hard enough idk. personally i just view it as mostly a marketing decision LMAO