dblack sorry, late reply but:
The short answer is economics.
The better question in my mind would be why we want to limit it. It is, after all, a limitation on free movement and free association. There are good reasons to do so such as maintaining culture and limiting the disruption that immigration naturally brings with it. However, those negatives only come about if we are allowing to much immigration to fast and I see no indication whatsoever that we are at that level.
As far as why we would want it, basic economics. Labor is a commodity and it is by far the most valuable you can have in any nation. Immigration does increase our economic success as virtually every single study on the matter seems to conclude.
I would, of course, prefer changing the system so that we have an immigration policy that is more in line with our economic needs and geared at brining in more capable individuals BUT no one banging on about immigration or a single bill ever introduced is actually trying to replace our non-existent immigration system. Instead it is only about stopping immigration rather than putting a reasonable and effective program in place.
I would support ending birthright citizenship (with the proper channels as it IS in the constitution) if those putting it fourth actually were serious and had a real immigration program with real goals in mind. Unfortunately, that represents, essentially, no one.
