Why have we made it so hard to immigrate?

I remember learning that immigrants used to come through the Ellis & Angel Islands and were given many tests before they could get in, but that was it if they were healthy enough and could read they were allowed in, although their names were sometimes changed and were often kept from coming in if they were from undesirable countries. So I grew up thinking that was terrible and that we by now we would have made it much easier to immigrate. Yet I come to find out it is actually harder to immigrate than it was then and that it can take many years to immigrate here unless you have an incredible skill or rich. So I wonder why is it so hard to immigrate in this day and age when America has a more liberal attitude about differences.