Why did こなた and そなた fall out of use but not あなた?

Question

Modern Japanese (by this I mean 標準語, I do not know about dialects) still uses あなた, yet both こなた and そなた seem to have fallen out of use. Confusingly, こちら and そちら are still very common. What happened to こなた and そなた, is there a specific reason why they fell out of use?

Answer 1

The original meanings of あなた/そなた/こなた are "that/this way", and these original meanings have fallen out of use, regardless of whether it's あ, そ, or こ. Today, we always say あちら/そちら/こちら.

On the other hand, あなた as a personal pronoun started as a highly respectful way of referring to someone. To convey respect, the word had to contain あ to express the "sense of distance". (This I think is somewhat similar to the English phrases "their highness" and "your highness" for expressing psychological distance; no one says "this highness" or "my highness", and you use it regardless of the person's physical highness.) So this is when the divergence occurred and あなた became special. Although こなた and そなた also developed derivative usages as second-person pronouns, their meanings were hardly any more than "this/that person", and they never carried a respectful feeling. そなた was seen as a condescending version of "you", primarily used towards someone of lower status. In samurai dramas, you can hear a higher-ranking samurai addressing his subordinates or commoners as そなた.

Although あなた was highly respectful at first, over time, あなた became less and less respectful, following the law called 敬意逓減/pejoration, until it finally became just another formal way of saying "you".