Why must meat be cooked to its minimum internal temperature?

Because of bacterias.

Explanation:

Foods have a safe minimum cooking temperature to be healthy and safe to our body, avoiding serious diseases. Different meets have different temperature to be cooked and ingest, this way you are going to destroy dangerous pathogens.

All raw meat contains bacteria that can be harmful to our bodies. Different types of meat have different temperatures to which we have to cook it to kill this bacteria, so we can eat it without getting sick.