Why does the brain need a constant supply of oxygenated blood?

Because braincells are very vulnerable to oxigen-shortage. Many other cells can recover after several minutes without fresh blood, but in normal temperatures your brain's chances of full recovery drop to zero in just 4 minutes.

Because body cells need oxygen for respiration and blood for transporting wastes away from the brain.

oxygen fuels the.chemical reactions that make everything work .. a muscle might be resilient enough to recover from starvation .. your brain will go into damaged retard mode and lose information if starved ...