The main reason as to why it's pronounced as "nihon" is because this is how it is pronounced in Chinese. In a missive sent to China back in the 600s, the emperor referred himself as the "Emperor of the Land of the Rising Sun", written as 日本. It was originally pronounced as "Hi-no-Moto" by the Japanese people, but in Chinese, while they share some symbols, the pronunciation is different. The deformed Chinese pronunciation as heard by the Japanese was "nippon", which then became "nihon", and they slowly started to adopt that reading, instead of the original "hi-no-moto", maybe because it was easier to pronounce, or it was more prevalent when trading, as China was the biggest trading partner at the time.
That's it for the history lesson. A lot of words were imported from the Chinese language and because of that, a lot of them won't make sense with the Japanese on'yomi and kun'yomi reading, so you can expect quite a lot of stuff like this. It's also the reason why you'll see some association of kanjis that make sense, while others don't (昨日 (previous+day for yesterday), 今日 (now+day for today), but 明日 (bright+day for tomorrow).
October 8, 2014 at 7:01am