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I think a lot of that has to do with the fact that the possibility was such a big thing in KH2. There were so many not-so-subtle hints in that game that I think a lot of fans thought that it would happen eventually. But then four more games happen, it's barely been touched upon again which is why I think the fanbase is now divided since all the games released after KH2 built more on friendship than romance.
You have just said it yourself: "Possibility", just like for a myriad other possible pairings there were many hints that could be taken as romantic if you tick that way or want to interpret it like that.
There were/are however also enough people who didn't/don't interpret it the romantic way.
Granted, some of the hints especially in KH2 were pretty blatant especially on the SoKai-front, but that can be also attributed to the difference between western audiences and japanese when it comes to physical contact and display of physical closeness. The westerners often tend to imply romantic interest everywhere while it can in the same context mean everything from just a very close friendship, sibling-like affection or simply a sort of admiration.
The fanbase was always divided especially on that topic, not to mention all the shipping madness with this series gets really out of hand more often than not.
I remember when we got the first substantial trailers for BBS the Terra/Aqua, Aqua/Ven and Terra/Ven shippers were already running rampant before the game was even released.
Wasn't Kairi brought up again (not sure if thats another hint) In KH: 358/2 Days since Xion looked like Kairi due to the fact that she was Sora's Strongest memories, I mean afterall Riku did say "To Sora, she's someone very special". I know "Special" can mean anything.
It can of course be seen that way, because it is Nomura's intention that you can see it in different shades. You can also argue that, to Sora, Riku, Donald and Goofy are also people very special and that amount of people is rising the further the series goes on.
The main reason Xion looks like a black-haired Kairi (which she only does to Roxas, at least only that is confirmed btw, everyone sees her different) is that Sora's memories that had to do with Kairi were the ones that were messed the most with by Naminé, and those were thus the first that could be absorbed by Xion's inherent memory leeching ability.