It depends on the application. In some cases yes in others no.
The Evening Halter Dress-Maternity has zippers not buttons
They made buttons first.
Buttons!
You don't have to replace zippers like you have to replace buttons
Buttons or the hook and eye.
People made zippers because they are easier to use and more efficient than buttons.
Buttons, straps, and strings.
They used buttons.
Buttons, zippers, pins.
You want to choose tops without buttons, hooks or zippers; pants or skirts with elastic waistbands or velcro closures, outer wear should also have velcro rather than buttons or zippers.
Buttons and zippers
It was probably done to save money at the time because World War 2 was going on at the time so they switched to buttons. Another answer: Zippers may have been switched to buttons so that the metal saved from not having zippers could be used in the war effort.