Why? Because in the Bible, leaven can denote sin or wickedness. Jesus was perfect, and his human body was without sin. He would offer that perfect body as a propitiatory sacrifice.
(‏ مرقس ١٤ :‏ ٢٢‏ )‏ بِهذِهِ ٱلْكَلِمَاتِ كَانَ يَسُوعُ يُشِيرُ إِلَى جَسَدِهِ ٱلْحَرْفِيِّ ٱلَّذِي كَانَ سَيُضْرَبُ وَيُضَرَّجُ بِٱلدِّمَاءِ .‏