At these places it's by choice. More often than not, these new immigrants with good credentials back home come to Canada under the guise of thinking that they'll have a better life, only to arrive and find out that the Canadian system doesn't actually recognize their degrees or experience from back home.
That's right.
Canada's immigration system demands only the best people with certified training and university educations be allowed into this country, but when they get here they find out no employer will hire them because they don't recognize those same credentials. I've met several Asian MPAs who were all fully trained and licensed as legit professionals (think our RMTs) in China, but when they came here they couldn't get a job in a legit spa and thus are now working in rub and tugs.
On the other hand, some also come to Canada to make a quick buck. Incalls in Canada pay 2-3x more than what they would get in Hong Kong (I've been to them there), so they come here (legally and illegally) to work a few months to a few years, then go home with enough money to buy a condo in Hong Kong, and if you know the real estate values there you'll know that it's a lot more money than the average Canadian can afford.