I personally actually think the dropped "hints" are about evenly distributed between both, especially when taking into consideration the whole series and not just KH 1 and KH 2.
The primary reason Sora X Kairi is seen as the more "probable" one inside out in general is because that is the hetero-option.
If Kairi would be a guy and Riku a girl (or Sora female), everyone would point towards Sora X Riku being the more probable one. It's something that is simply more engrained into most people's minds, similar to the notion that people of different genders simply cannot be just friends but have to have something romantic going.
Shipping is ok as long as it doesn't go overboard and becomes intruding to the whole fandom. You're lucky to not have entered the fandom around when KH 2 was still freshly out as I remember back then in 2006/2007 especially the Axel/Roxas shipping could sometimes get quite intense to the point where it did become somewhat unsettling.
Not to mention, since KH is a series that loves to keep all the romantic stuff regarding the original characters vague, the shipping wars themselves became sometimes so "hot" that several forums and fansites eventually banned shipping-talk completely.
That there are no (or very few) canon non-hetero relationships in entertainment media in general has also much to do with the general mindset of the majority of consumers and creators as many of the latter group are simply too fearful to include an open non-hetero-relationship because of possible backlash from "moral guardians", religious zealots or possible poor success of the work because of it.
Eh, I just don't really see any hints for SoRiku. I'm totally not against the homosexual agenda, I'm just saying that there really wasn't anything that hinted anything more than two bros in my eyes. I guess the closest we got to this was the scene before Riku dives into Sora's heart to beat the Armored Nightmare, but even then I still see that entire scene as definitely platonic.
P.S. TOTALLY love homo relationships in popular media, however KorrAsami for life yo ;W;