Because it affects the trade

It works once or twice. But once word spreads (and our friends at the media will be happy to spread it) that extortion is the business, it will mean either of two things:

a) Rich country foreigners no longer go to the countries and cities rumoured to be extortion havens.

b) Rich country foreigners no longer stay at the main, not-so-poor cities and tourist venues, but venture deeper in the country, hoping to find a community poor enough to accept prostitution but not yet engulfed in the extortion cycle.

It's pretty much like it is with drugs or with prostitution in the West, involving upmarket citizens. If you have a prostitution ring, or a drug distribution business, catering to rich and / or famous Hollywoodians, businessmen, or politicians, then yes, you'd be able to make more from extorting one of them once than you probably do from continued regular service. And then as soon as word spreads, you're out of business.

In the long run, pimps know it's better to continue providing the service to the GLitters of the world than to kill the chicken...