According to "the great depression," why was the prosperity of the 1920s "unsustainable"?

The 1920's was a time of sensational economic growth and progress. Mass produced goods such as clothing, radios and cars were in abundance. The stock market boomed and most Americans benefited. But there was also economic inequality, thus the prosperity of the 1920's was unsustainable.

In this sentence, hard is the adverb because it is modifying the verb.