Final answer:
The sparse population in the western parts of North America is due to harsh climates, vast mountainous and arid regions, and seismic risks that make the environment challenging for dense human settlements.
Step-by-step explanation:
The western part of the United States and Canada are sparsely populated primarily due to vast mountainous and arid regions, which create challenging conditions for large-scale human settlement. The harsh climate and extreme weather conditions, especially in the northern areas, also contribute to the low population density.
Regions that are prone to seismic activity, particularly near the Pacific Coast, further discourage dense population due to the risks of earthquakes and volcanic activity. Additionally, the economic core of North America, with better climate for agriculture and more diverse economic opportunities, has traditionally been located in the northeastern United States, making the west less populated by comparison.