The reason is quite obvious. A Christian does not care whether we Muslims believe the Bible to be corrupt because he has faith that it isn't, regardless of how much irrefutable evidence we present to the contrary. But, if a Christian does have faith in the Bible, then he has to accept the verses that refer to the Prophet Muhammad عليه الصلاة والسلام and إنشاء الله showing how the Bible points towards Islam will lead towards more questioning of the viewpoint held by many Christians about the divinity of 'Eesa عليه السلام and other strange beliefs that form the core of Christian belief but are never explicitly mentioned within the Gospels.