Why「こんにちは」instead of 「こんにちわ」?

Today I recieved a question from my student in the Beginner Course.

She asked me why, in my email, did I write "konnichiwa' (hello) as「こんにちは」instead  of 「こんにちわ」. 

This is a good question.

I taught my students  in my first lesson that to say 'A=B ' in Japanese, it's 'A wa B desu'. And when you write this sentence pattern in hiragana, it should be 「AはBです」. Please remember this rule and I will explain why this is so when you become an advanced learner.

For instance, 「わたしはともきです」,「これは時計です」.

Konnichiwa comes from the phrase "今日はいかがですか?(Konnichi wa ikaga desu ka?)" meaning "How are you today?", where "は" is used as a topic maker. 

The explanation is the same for "こんばんは" (konbanwa), which means "Good evening."

In modern Japanese, 'today' is 'kyou'; but it was ' konnichi ' in old Japanese ~.

Note: 
こんにち/今日(konnichi) = this day/today 
こんばん/今晩(konban) = this night/tonight