Why was 'structural' chosen to mean 'structural unemployment'? Source: Economics (3 ed, 2014) by N Gregory Mankiw, Mark P. Taylor


  structural unemployment [=] unemployment that results because the number of jobs available in some labour markets is
  insufficient to provide a job for everyone who wants one


Please help me dig deeper than the definition above, the meaning of which I already understand and so ask not about. Instead, I don't understand why structural was selected for the above.  Why not adjectivize this type of unemployment with another word? 

This definition (below) doesn't answer my question; all unemployment is caused by structure of the economy. So what's so structural about structural unemployment? 


  
  c. :  involved in or caused by structure[,] especially of the economy
Cyclical unemployment is named after the business cycle, as in the part of unemployment that varies with the business cycle. Structures are things that last a long time, so structural unemployment is the part of unemployment that is driven by features of the economy, like structures, that last a long time. 

But as you intuit, surely the length, frequency, and other details of business cycles are driven by such structural features. That's almost certainly true, and major reason why structural change and good policy are thought by many to explain the great moderation in business cycles.