Answer:
Depends where and when, but generally because they upset the prevailing culture which people see as a threat to their ways of lives and livelihoods - such as willingness to work longer hours for less money in poorer conditions and ability to save money while doing it.
I knew a Chinese immigrant family that was resented for being able to afford to buy a house within five years of their arrival. No one considered it was because the whole family sacrificed quality of life to work 12 hours a day, six days a week running a Chinese restaurant when other workers got overtime pay for anything over 7 hours a day and a 5 day week.