It's most likely due to newer, more strict tax policies. U.S. citizens can be required to pay taxes regardless of which country they live in for periods of time. But the law also allowed the U.S. government to go after foreign bank accounts, which prompted many foreign banks to drop their U.S. customers.
"An increasing number of Americans appear to believe that having a U.S. passport or long-term residency isn't worth the hassle and cost of complying with U.S. tax laws."
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