It's the same for people that like Riku/Sora, Axel/Sora, Axel/Roxas, Riku/Namine, Namine/Kairi etc. etc.
It's the charm~
I think.
And it's cute. I know most of Sora's feelings for Naminé were fake, but the pairing is still cute and that's what attracts people.
And you know how fangirls are...