The temple is holy ground just like the temple of Solomon in the Bible. Only certain people are allowed inside. Just like with Solomons Temple in Jerusalem during the time of Christ.  
 
So only members who are in good standing and are recommended by there local priesthood leader (called a Bishop) are allowed inside to protect the sacred nature of the temple.  
 
You are welcome to tour all meeting houses and the temple grounds! Also when a temple is completed or renovated they have an open house where the public can tour the temple before it is dedicated! This it the only time the public is allowed inside a temple.