Why is the adjective before the noun?

Although French adjectives mostly follow the noun, some French adjectives are always used in front of the noun and others are moveable (with the meaning sometimes being a little different before or after the noun). There are patterns to which adjectives go before nouns, and as to how meaning may change for moveable adjectives, but remember that most grammar rules have exceptions. 

There are a number of lessons to be found here on adjectival positioning - the linked lesson is a good place to start as it covers your immediate question directly :

 Position of French Adjectives - Short and common adjectives that go BEFORE nouns