The reason mammals are not very good at tolerating hypoxic (i.e. low oxygen) environments, is because the brain relies heavily on oxygen for metabolism. It is so stingy, it uses about 20% of the oxygen in the body to make ATP. Without oxygen, the brain has to rely on glycolysis to make ATP and this process is not good at meeting the energy requirements of the neurons, ultimately resulting in cell death.
Did you know that goldfish, Carassius auratus, have a remarkable ability to tolerate anoxia without nerve damage? In fact, they can also survive in low oxygen water for weeks to months. These fish are able to survive in oxygen-free water by altering GABA neurotransmitter signals to help the neurons become less excitable, meaning they need less ATP (and hence less oxygen). Pretty cool.
N Hossein-Javaheri, LT Buck. GABA receptor inhibition and severe hypoxia induce a paroxysmal depolarization shift in goldfish neurons. Journal of Neurophysiology. 125(2): 321-330, 2021.