Scientists believe that the Earth's early atmosphere lacked oxygen because photosynthetic organisms did not yet exist. Hope this helps and have a great day!
During photosynthesis, there is consumption of carbon dioxide and water to produce glucose and dioxygen. Glucose is the "fuel" of its growth. Cellular respiration will consume some of this glucose and oxygen to form oxygen and water.
So, if the atmosphere was lacking oxygen, it's simply because there was no photosynthesis producing oxygen.
