Why were they called the "Old Immigrants"?

Old immigrants came to the U.S. and were generally wealthy, educated, skilled, and were from southern and eastern Europe. New immigrants were generally poor, unskilled, and came from Northern and Western Europe.

Explanation:

the monroe doctrine is the best known u.s. policy toward the western hemisphere. buried in a routine annual message delivered to congress by president james monroe in december 1823, the doctrine warns european nations that the united states would not tolerate further colonization or puppet monarchs.