With no Republican prepared to embrace an amnesty-like program that Ronald Reagan implemented in the 1980s, immigration in America has become a stalled and divisive topic. Throw in Donald Trump and his race-baiting "birther" campaign, his policies towards Muslims and his rejection of DACA for young immigrants, and it's easy to understand how immigration and race relations in America have become absolutely toxic.
It all adds up to an environment where immigration policy in America has become a real but unnecessary mess.
Below is an abbreviated overview that, incredibly enough, also provides a very helpful visual that does a good job of explaining why undocumented immigrants from Mexico, and other parts of Latin America, can't actually "get in line" and "do things right." Simply put, years of political brinkmanship and moral cowardice has created a cruel cul-de-sac of rejection for the vast majority of America's undocumented populations.