partly population concerns - while Amwerica is a big, wide, empty country you all want to live in very crowded little communities. Increasing immigration causes more pressure on those communities for things like housing and traffic.
Then there's the economic issue, while the wild west had no social care benefits, today you have many. So every new immigrant either has no job and gets benefits, or has a job and pays his own way but helps to displace another worker who then ends up on benefits.
In the UK we see this a lot, while immigration has increased dramatically, the number of jobs has increased relatively slowly, so we have 6 million immigrants but 2 million unemployed. Our health and education systems have not been funded accordingly though, and are showing signs of collapse. Hence, immigration is a good thing, but only to a point - not as an unlimited influx.
Its probably entirely linked to the rate of immigration overall, in the old days when we had few immigrants being drip-fed into the system things were OK, now we have a flood people are getting concerned.