Postby QuackingShoe » March 2nd, 2010 1:10 pm
Because there were much, much more important objects that needed to be represented with the shape of a circle than the word 'circle' did. Language develops in a certain order - the things most inherent to human survival and experience take priority when choosing words and, in this case, shapes. Specifically, 'mouth',口, was much more important, so it was a circle. Additionally, 'the sun' is drastically more important, and the modern kanji for 'sun', 日, evolved from a circle with a dot in it. And 'eye', 目, was a circle with another circle in it, afaik. Etc.
The concept 'a circle' comes in somewhere near dead last.