There are two reasons. First, Scripture is defined as that collection of 66 books called the Holy Bible. Since the time is was accepted (canonized) it has been viewed as complete and nothing has been added to it.
Second, the gift of prophecy, as practiced in the church today, is much different from the office of prophet, wherein God appointed a man to speak for Him on Earth, often revealing new aspects of His plan. The last person who held the office of Prophet was Jesus (Hebrews 1:1-2).
Today the gift of prophecy is meant to illuminate Scripture, not create or modify it. For that reason it must conform to the Word of God as it’s already been revealed. In other words, it only helps us understand Scripture, it doesn’t become Scripture.