Why are there more organisms who need oxygen than those who can live without

Organisms, who can live without oxygen are greatly outnumbered by ones who need oxygen to live. Why is it so? Please give me two reasons.

E.g. something to do with evolution maybe?

This is only the case in environments where there is oxygen. Oxygen is toxic to strict anaerobes, so they now inhabit radically different environmental niches than aerobic ones--the bottom of the oceans (e.g. the iron-"eating" communities of bacteria and archaea that in about 1000 years will have completely devoured the Titanic), deep lakes, swamps, beneath the mud floors of these bodies of water, our large intestine, etc. In those environments it's the aerobes that are either nonexistent or in the minority.

The larger bio-diversity and complexity seen in aerobic environments is due to the relatively higher efficiency of energy transformation favored by evolution of aerobic metabolism as compared with its anaerobic counterpart.

Oxygen is a highly energetic element. When it is broken down from O2 to other forms of compounds like CO2 it releases a huge amount of energy. Many organisms use this plentiful abundance of high energy chemicals for their own energy. The Cambrian Explosion, which took place about 530 million years ago, is an explosion of the diversity of life on the planet. This was due to the new presence of oxygen in the atmosphere.

Basically Oxygen is too good an energy source to ignore for most organisms

Oxygen is there so obviously something would evolve to make use of it. The reason oxygen using organisms are so much more common is because oxygen is like rocket fuel to life.