The number of immigrants per capita in the US is not especially high. It's lower than in Switzerland, Australia, Canada, Ireland, Germany and similar to most Western European nations.

1. Because US is big. 2. Because it is more welcoming for immigrants. 3. Because of broken H1B system.

Because it had great (and I mean multi-generational enduring American Dream great) PR from after WWII thru at least the late 60's.

The post-WW2 soft power strategy by the US was awesome and extremely effective. Sadly, it seems that by the 70s the pentagon had become solely obsessed with the projection of hard power. I personally reckon this has a huge amount to do with the decline of the US's popularity around the globe in the subsequent decades.

As an outside observer and frequent critic of US foreign policy I would agree.