Etymology behind the phrase 恭喜发财 (Kung Hei Fat Choy) and its usage during Chinese New Year?
恭喜 (congratulations for joyous occasion) : &quot;Bless you&quot; is the standard opening for giving good wish to others, for example: 恭喜你大學畢業(congratulations on graduation from university), 恭喜你新居入伙 (congratulations on moving in to a new home). So &quot;恭喜发财&quot; (congratulations on getting rich) is just one of many things you can bless people with, no more common than &quot;恭喜你新年快樂&quot;

From https://zhidao.baidu.com/question/591040765.html
In 1826, an English man went to Guangzhou and worked in an American trade company. He wrote a book called 《洋人在广州》(Foreigners in Guangzhou). In that book he mentioned the Chinese who traded with the company would greet them with the Chinese phrase &quot;恭喜发财&quot; during the Chinese new year. (the common goal among merchants is making money after all). After the publication of the book, greeting people with &quot;恭喜发财&quot; in Chinese new year became more popular in Guangzhou. And presumably, this practice spread to the rest of the country too.

I suspect the reason &quot;恭喜发财&quot; is so popular is partly due to gambling was a national pastime in China at the time. Common folks could only openly gamble  during the new year period. Wishing someone getting rich implied wishing him good luck in gambling, beside congratulate the other on making a lot of money from the previous year (you wouldn't find out how much you made until you'd done the year end accounting)
I also suspect the reason &quot;恭喜发财&quot; is particularly popular among Cantonese,  is partly due to a jingle (順口溜): &quot;恭喜发财，利士來&quot; (congratulations on getting rich, give me red pocket money)