Answer
Organisms which evolved in a reducing atmosphere would be poorly prepared to cope with oxygen. They would likely be badly damaged by oxidation when oxygen appeared in the atmosphere, leading to extinction of many of them.
Work Step by Step
The key here is to see the difference between the early atmosphere (review Oparin-Haldane) and an oxygen-containing atmosphere. Think of what oxidation does today (and the importance of antioxidants in our diets), and how unprepared cells from a reducing atmosphere would be to deal with such an atmosphere.
