The short answer to why the title of a book may not be protected by copyright is that the law says it may not, or rather, excludes titles from the list of things which are subject to copyright protection.

The longer answer is that it would be altogther silly to allow anyone to copyright the title of a book, in that this would (theoretically) prohibit a cataloguer from listing it, a bibliopole from advertising it, a reviewer from reviewing it, and so on.