Final answer:
Roman Catholics are clustered in the U.S. Southwest primarily due to the historic and ongoing immigration of predominantly Roman Catholic Hispanic populations.
Step-by-step explanation:
Roman Catholics are clustered in the U.S. Southwest primarily because of migration due to the presence of a strong Hispanic population in those regions. This clustering is a reflection of historical immigration patterns, with Roman Catholic dominance in areas like the Northeast and Midwest pointing to 19th and early 20th-century immigration from Roman Catholic countries in Europe. Additionally, the pattern has been influenced by more recent trends, including the immigration of Hispanics, which has been a contributing factor to the Catholic presence in the Southwest.
Immigration has played a significant role in shaping religious demographics, with cultural traditions leading to communal clustering. This is also seen in how religious institutions, such as Catholic churches that were established early on, can attract more people of the same faith through a process known as chain migration. Thus, the historic and ongoing immigration of Hispanic populations, who are predominantly Roman Catholics, is a primary reason for the concentration of Roman Catholics in the U.S. Southwest.