Kimchi is derived from chimchae,which means preserved vegetables with salt, and as time went by,the word had been changed: dimchae-kimchae- kimchi. And now it'scalled kimchi.
 

Kimchi in early days was simply pickled vegetables, However, from about 12th century various flavors were used to make special taste of Kimchi, and in 18th century, red pepper started to be used.
Red pepper contains capsaisin, which makes hot taste and helps the function of salt.
It is assumed that the reason why red pepper was used a lot more than others was to save salt since it was so rare in old days.
Also, in 19th century,cultivating of Korean cabbage for kimjang: kimchi for winter came to be pervasive, Consequently, the traditional kimchi as same as today had been holding its shape.
 

There are three major reasons that kimchi had been developed as fermented food specially in Korea, while there are also many kinds of fermented food using vegetables as major ingredient all over the world.
First, Koreans, whose main industry was agriculture, enjoyed vegetables.
Second, high technology enabled to preserve different fish with salt could
play a role in using fishes as main ingredient widely, and especially cabbages for kimch were produced and provided widely from coast to coast.