You're asking the question the wrong way round. You should be asking "Why do people spell こんにちは *konichiwa* rather than *konnichiwa*?" and the answer is "because unfortunately they don't seem to know any better"...
– EarthliŋMay 30 '18 at 18:56
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It's actually konnichiwa(こんにちは). We don't say konichiwa(こにちは).