gekota
That is why.
It's like there's some strange curse on using the four Railgun characters' given names. Mikoto calling Kuroko Kuroko is the only time their names get used with any sort of regularity. XD
<Snipped quote by Raineh Daze>
I blame differing cultures. (IC, of course)
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That'd be part of it, but the sheer persistence of it is impressive.
Well, the family name gets used more often for politeness. Note that since Misaka and Kuroko are close, they are on first name basis. While Uiharu is friends to them, she is more polite and would usually call them by their family names.
EDIT: Also Misaka prefers Misaka to be used since Misaka likes Misaka better, said Misaka to clear up any confusion.
@Flamelord Feel free to post. Kuroko's not going to be instantly back. :T