The Southwest of the United States is one of the least densely populated areas in the country. The reasons for that are mainly the geography and climate of the area.
The Southwest is predominantly arid and semiarid. The rain is very rare, and the temperatures are very high. The soil is not suitable for agriculture.
This region also has mountains, plateaus, deserts, all of which are pretty inhospitable for human life, so understandably, the people avoided this place for settling down.
Life began on Earth at least 3.5 to 4 billion years ago, and it has been evolving ever since. At first, all living things on Earth were simple, single-celled organisms. Much later, the first multicellular organisms evolved, and after that, Earth's biodiversity greatly increased.