Probably for the same reason it hasn’t “eradicated” influenza, measles and other infectious diseases. Actually, however, even if plague hasn’t been eradicated, it’s been brought under control to the extent that it’s extremely rare, and never the result of poor sanitation, malnutrition and the proliferation of the species that carry it.
Think again. See this prior FreeRepublic post and go to the source article.
The Mutant Genes Behind the Black Death
Quanta Magazine ^ | 10/6/15 | Carrie Arnold
Posted on ;PM by LibWhacker
http://www.freerepublic.com/focus/f-chat/3346880/posts
Once this bactria changes a gene or two again as it has in the past, it could get out of control and almost immediately produce yet another pandemic far beyond the mortality experienced by all the past human plagues combined.