There were lots of causes but I think The Immigration Act of 1965 was key. It was designed by Teddy Kennedy and his liberal cohort to multiculturalize America. Denying the country’s founding principles and history has been part and parcel of that fateful legislation’s goal.
1921 - The Emergency Quota Act, also known as the Emergency Immigration Act of 1921, the Immigration Restriction Act of 1921, the Per Centum Law, and the Johnson Quota Act (ch. 8, 42 Stat. 5 of May 19, 1921) restricted immigration into the U.S. Although intended as temporary legislation, the Act "proved in the long run to be an important turning-point in American immigration policy" because it added 2 new features to American immigration law: (1) numerical limits on immigration from Europe and (2) the use of a quota system for establishing those limits. The Emergency Quota Act restricted the number of immigrants admitted from any country annually to 3% of the number of residents from that same country living in the U.S. as of the U.S. Census of 1910. Based on that formula, the number of new immigrants admitted fell from 805,228 in 1920 to 309,556 in 1921-22. The act meant that only people of Northern Europe who had similar cultures to that of America were likely to get in. The reason given was that the American government wanted to protect its culture when this act was introduced, however some felt it was mainly a response to Eastern European Jews fleeing persecution that had been growing since about 1890.
1965 - The Immigration and Nationality Act of 1965 (Hart-Celler Act, INS, Act of 1965, Pub.L. 89–236) abolished the National Origins Formula that had been in place in the U.S. since the Emergency Quota Act of 1921. It was proposed by U.S. Representative Emanuel Celler (D-NY: His paternal grandparents and maternal grandmother were German Jews. This was the culminating moment in Celler's 41-year fight to overcome restriction on immigration to the U.S. based on national origin.), co-sponsored by U.S. Senator Philip Hart (D-MI) and heavily supported by U.S. Senator Ted Kennedy (D-MA). The Hart-Celler Act replaced the EQA with a preference system that focused on immigrants' skills and family relationships with citizens or U.S. residents. It marked a radical break from the immigration policies of the past. The law as it stood then excluded Asians and Africans and preferred northern and western Europeans over southern and eastern ones. At the height of the civil rights movement of the 1960s the law was seen as an embarrassment by, among others, POTUS #35 JFK, who called the then-quota-system "nearly intolerable". Some historians thought that JFK saw a chance for retaliation in response to the anti-Irish Catholic bigotry by WASPs he encountered as a younger man. After Kennedy's assassination, POTUS #36 LBJ signed the bill at the foot of the Statue of Liberty as a symbolic gesture.
In order to convince the American populace - the majority of who were opposed to the act - of the legislation's merits, its liberal proponents assured that passage would not influence America's culture significantly. POTUS #36 LBJ called the bill "not revolutionary", SoS Dean Rusk estimated only a few thousand Indian immigrants over the next 5 years, and other politicians, including Senator Ted Kennedy, hastened to reassure the populace that the demographic mix would not be affected; these assertions would later prove wildly inaccurate. In line with earlier immigration law, the bill also prohibited the entry into the country of "sexual deviants", including homosexuals. By doing so it crystallized the policy of the INS that had previously been rejecting homosexual immigrants on the grounds that they were "mentally defective" or had a "constitutional psychopathic inferiority".
After ethnic quotas on immigration were removed in 1965, the number of actual 1st generation immigrants living in the U.S. eventually quadrupled from 9.6 million in 1970 to about 38 million in 2007. Over 1 million persons were naturalized as U.S. citizens in 2008. The leading countries of origin of immigrants to the U.S. were Mexico, India, the Philippines, and China. Nearly 14 million immigrants entered the U.S. from 2000 to 2010. Family reunification accounts for approximately two-thirds of legal immigration to the U.S. every year. The number of foreign nationals who became legal permanent residents (LPRs) of the U.S. in 2009 as a result of family reunification (66%) exceeded those who became LPRs on the basis of employment skills (13%) and for humanitarian reasons (17%).