Originally, they had this flag from 1975-1992:

Clearly, this version was related to Guinea-Bissau, to which Cape Verde had close ties. Both were Portuguese colonies and their independence movements were intertwined. There were even ambitions unifying the two countries.

Wikipedia on the relations: https://en.wikipedia.org/wiki/Cape_Verde–Guinea-Bissau_relations

It never happened though, and Cape Verde broke a way after a coup in Guinea-Bissau in 1980, and went its own way with its own flag, which I personally find more appealing. It is more distinct and free from the coat of arms.