Question

How did Yokohama become known as a "boomtown?"

Answer 1

Yokohama was one of the first cities to be open to foreigners following the treaties. So, it was a hub of interaction among Japanese who had been nationally secluded for 200 years and foreigners

Answer 2

Interestingly, despite the openness, the port was still positioned so that foreigners were isolated and observable by the Japanese

Answer 3

Also it magnifies "sudden prosperity" due to foreign interaction.