There are several reasons for this trend. One factor is the increasing global demand for skilled workers. As the US economy has become more knowledge-based and technology-driven, employers are seeking workers with specialized skills and expertise. This has led to an increased demand for foreign-born workers who possess these skills.
Another factor is the US immigration policy, which gives preference to immigrants with skills and education that are in demand in the US labor market. The Immigration Act of 1990 introduced a point system that favors applicants with higher levels of education and job skills. This system has helped to attract highly skilled immigrants who can contribute to the US economy.
Finally, it is important to note that the educational attainment of legal immigrants varies by country of origin. Some countries, such as India, China, and the Philippines, have a large number of highly educated immigrants who come to the US for work or study. These immigrants contribute to the overall increase in the average educational attainment of legal immigrants in the US.