Googled "what is a Bible Commentary" and got the answer to your question:
- Web definitions
- Exegesis is a critical explanation or interpretation of a text, especially a religious text. Traditionally the term was used primarily for exegesis of the Bible; however, in contemporary usage it has broadened to mean a critical explanation of any text, and the term "biblical exegesis" is used ... http://en.wikipedia.org/wiki/Bible_commentary
Definition: A biblical commentary is a written systematic series of explanations and interpretations of Scripture. They are written by some of the most knowledgeable theologians in church history. Through a personal narrative, they provide deeper understanding and insight into the Bible, and can be valuable tools to assist both casual reading and serious study. http://christianity.about.com/od/glossary/g/Commentary.htm
As we all know, the WTBS likes to explain the Bible by jumping from verse to verse. A commentary, being a line by line examiniation of a text, does not allow this type of interpretation. Studying the Bible text using a line by line sytematic approach would destroy the Watchtower's interpretations.