It is a mystery. I don't think it's some grand thought-out non-poaching agreement, though. I think you have to break it down to domestic politics. The left wan

It is a mystery. I don't think it's some grand thought-out non-poaching agreement, though. I think you have to break it down to domestic politics. The left wants to maximize multiculturalism. This has become a sort of fundamental value to that tribe (rather than a means to an end), and their fundamental anti-value is racism/xenophobia/intolerance of anyone not white-Christian. So that tribe will resist any hint of an attempt to selectively immigrating people from culturally or racially similar countries, because such immigration actually threatens to reduce multiculturalism, which to them is a fundamentally evil thing. So they'd call it racist. In addition, even just choosing people for wealth or intelligence or English skills, you'd end up with a big cultural/racial skew towards white people (compared to the world population), so the left would call it racist for the same reasons. Those on the right might be happy with such immigration, but it's too difficult of an argument to make and it's too easy for the left to call them racists if they suggest anything like it. So as a proxy, they tend to argue for reducing all immigration. I must be missing a lot of the details but I think it's some sort of political compromise/deadlock like this.