Because you are polish and trying to do it legally. Mexico thinks it has specialy rights and apparently some here in our govt. do as well.
Actually It's not so simple to do it legally. It's very HARD to get immigration VISA to USA.
I think immigration is pretty easy --- the USA takes in far more legal immigrants than any other nation does. We've doubled our population in less than 50 years --- through massive immigration. How much easier should we have to make it?
But you might be right --- it's very easy for Mexicans to immigrate here -- which is why they are the overwhelming portion of our legal immigrants --- but maybe not very easy for others who might have more education, work ability, and knowledge of our language.