I think TAV's success in the poll speaks less to their popularity here and more to... the rest of the options just not being very good xD
That and some bits may also be counting in more of the potential in them than what was actually done with them so far.
When it comes to TAV, my to-go reference is almost always the issue that both the opening of 0.2. and arguably the ending of KH III with the part where Chirithy joins them tends to show TAV in a better light than their supposed main appearance in BBS itself.
Out of the other options, ironically the "Destiny Trio" never really felt like a true trio almost from the start because it is is almost always about Sora's individual bonds with the other two or those other two indirectly "competing" over Sora's attention which vast parts of the fandom has picked up to an even more obvious degree.
The games themselves also do this with "side" games often focusing completely on Sora's bond with Riku with Kairi sometimes completely absent while "main" games tend to pop up Kairi only to toot more in the performative romance horn while Riku is reduced to a third wheel (but he's at least present compared to Kairi in "side" games).
RAX has a similar problem with Roxas being the center and on which bond the focus lies depends on the writer.
That being said, while it is not good
there are attempts to at least partly build a sort of small bond between Axel and Xion, a little bit in Days and a little bit more in KH III.
Like said, not good
but certainly more effort than Kairi and Riku got so far.
The "Disney trio" is a neat concept but so far we haven't exactly seen anything
on this trio because DG are either with Sora or bystanders while Mickey is traveling solo (and 95% behind the scenes) or with Riku which forms a duo of sorts. We haven't yet any substantial material to work with for MDG.
The so-called "Lost trio" is a fan-invention that has no actual basis in the series itself.
It is just another visual repeat of Destiny Trio with Repliku being the "Riku", Naminé the "Kairi" and Vanitas the "Sora" solely because of appearances and not because of personalities and traits.
Eraqus, Xehanort and Yen Sid were indeed peers to some degree, but the latter was never actually part of their closer circle so Eraqus and Xehanort also form more of a duo just like Mickey and Riku and, in the past before the introduction of Xion, Axel and Roxas were.
Interestingly, since BBS one could also take Lea and Isa as another duo until KH III shoved in the new plot device "Subject X".
Seeing this one could be tempted to see a pattern there, one that is as unnecessary as Nomura's constant "surprise twists" just for the sake of having them even if they hurt the Kh-verses internal logic more than they expand it.
Union X and Dandelion Trio I would also catalog under the category fan-invention because so far I don't see the story actually pointing towards these at all. Ephemer was almost completely absent during the time the Player avatar spent with Skuld and when they were with Ephemer there was no Skuld. Lauriam and Elrena have exactly one
scene where they talk about
Strelitzia but there is nowhere any hint that they used to hang out together nor that Lauriam even knew Elrena that much before he sought her out to gain info on his sister.