In the centers of north america and asia, large regions of land have relatively dry climates. this lack of rainfall results mainly from which factor?

The simplest answer here would be the distance.

By distance we mean specifically the distance the areas have from the oceans. The central parts both North America and Asia are far enough from ocean for them to have relatively dry climates. This is because the the moist air from the ocean is prevented by the distance to reach these regions.