It's the same reason the particle *wa* is spelled は in kana. The sound changed, and when they revised kana spelling to better represent modern pronunciation, they kept a few things like は spelled the old way.
– Jul 10 '15 at 00:52
1 Answers1
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Are you are talking about 今{こん}日{にち}は?
I guess you can see everything you need by looking at ruby characters above.