The Sociology Department closed in 1989 for various reasons.  You can read more about the history of the department in these articles:

Sociology at Washington University in St. Louis: History and Reflections, 1906-1989 David J. Pittman and Deirdre Boden

The American Sociologist , Vol. 20, No. 4 (Winter, 1989) , pp. 305-321 Published by: Springer

Article Stable URL: http://www.jstor.org/stable/27698490

History of Anthropology at Washington University, St. Louis, 1905-2012 David L. Browman

http://openscholarship.wustl.edu/books/1

Washington University's Pig Roast: Ending Sociology and Jeopardizing Liberal Arts Barbara Sherman Heyl

The American Sociologist , Vol. 20, No. 4 (Winter, 1989) , pp. 341-345 Published by: Springer

Article Stable URL: http://www.jstor.org/stable/27698495

In 2015, Washington University re-established the Department of Sociology in Arts & Sciences.  https://sociology.wustl.edu/

You can discover more about Washington University's history by browsing a guide to University Archives, http://libguides.wustl.edu/universityarchives