I have never seen better information compiled about the Word of Wisdom anywhere than in this e-book:
Steven Horne took the Word of Wisdom line by line and expanded upon it with what the modern prophets (like Hyrum Smith, Brigham Young, and others) said, and added other very valuable information. This is a comprehensive answer to the "why's," in my opinion.
You may or may not agree with all he says, but it's an easy read and I think he supports his research very well. We read a chapter weekly for FHE until we read completely through and we've tried to implement things as soon as we were ready. It's an easy read. It's free. Go for it!