Answer
The main reason is that USA is the land of opportunities
Explanation:
There is no free national health system in UK.
there is no respect of animals as compared to USA
USA is very wealthy and living standards are comparatively higher than UK.
Housing in the UK is very expensive and salaries are very low even for professionals so if someone is in a profession which makes moving to the US possible it makes financial sense for them to do so. They will be able to enjoy a comfortable lifestyle, their own home and plenty of disposable income in the US
The other two main reasons are marriage and money.
