As immigrants arrived from many nations, why did it become necessary to restrict who and how many came to america

Well, first of all everybody had to be checked so as to see whether they have any contagious diseases that would prevent them from being admitted into the country. Then, their backgrounds had to be checked so as to prevent criminals and possible terrorists from entering the US. Lastly, their numbers had to be limited so as to keep Americans from rioting if they start taking their jobs and just general space, which doesn't really belong to them yet.