Because the Quran is the last book remember.
The Hadith and later scriptures written is what by the Quran?
Whatever is explained to you once you believe jesus has returned will be the word of Allah to you because you believe in only man's testimony.
"We accept man's testimony, but God's testimony is greater because it is the testimony of God, which he has given about his Son. Anyone who believes in the Son of God has this testimony in his heart. Anyone who does not believe God has made him out to be a liar, because he has not believed the testimony God has given about his Son. And this is the testimony: God has given us eternal life, and this life is in his Son. He who has the Son has life; he who does not have the Son of God does not have life." 1 John 5:9-12