A city dependent on mono-industry has a risk of declining easily. For example, before 1971, Seattle was dependent on one airplane manufacturer. Once the company announced reduction of 60000 workers, the city dived into recession.
I'll show you another example. Osaka, recently depends on consumption of foreign tourists, was damaged economically when Kansai International Airport was forced to close after Typhoon caused flooding on the airport, and tanker hit the bridge connecting the airport and mainland. Though it just needed about 2 weeks to reopen the airport, the city was fortunately able to recover its economy soon.
But, the disaster told us how weak it is to depend on only one industry.