"Why" is sometimes impossible to answer with grammar...
When saying: "The (adjective) (noun)" The formula in Hebrew is: (from left to right)
If you do not include the second ה the phrase becomes: "the (noun) is (adjective)"
Examples: The book is new. - הספר חדש
The new book - הספר החדש
It may help if you think of the "is" that breaks up the "the" in the English equivalent. In the second English example "the" applies to both "new" and "book," but in the first example the "is" divides between them.... Hope that helps in some way.