Ohayou Gozaimasu and Konnichiwa

My question is more related to the Japanese custom rather than the word.

I understand that Ohayou is the word for morning that is usually followed with the word Gozaimasu as a form of politeness.

Then why is Gozaimasu is used for the morning as a custom, but not with the daytime greeting Konnichiwa?

First off: "ohayou" does not mean "morning". Morning is "asa".

Ohayou is derived from "hayai" (meaning "early" or "fast").

Gozaimasu is typical of a certain level of polite speech, but it isn't just a "politeness word" that gets tacked on wherever you want to sound polite. It is a verb.

Konnnichiha is actually an abbreviated greeting and I don't feel like looking up the rest of it just to tell you about it. Suffice it to say the two greetings as you encounter them share no structural/grammatical commonalities.

As Mike-san wrote, that's because of the etymologies of those words. "Gozaimasu" is a polite form of a subsidiary verb "aru", thus, "ohayō gozaimasu" originally means "o hataku aru (lit. it's early)". Whereas "konnichiwa" is from the begining of a sentence "Konnichi wa ohigara mo yoku(Today is a good day)", "Konnichi wa yoi otenki de (Today is a fine day)", "Konnichi wa gokigen ikaga desu ka (How are you today?)" or like that. "Konnichiwa gozaimasu" literally means "There is something today". It doesn't make sense as a greeting. That's the reason.