Because we Muslims believe that some previous scriptures have remnants of Divine Truth in them. For more on Prophet Muhammad, peace be upon him, in previous scriptures, visit:
www.answering-christianity.com/predict.htmAlso, what agrees with the Glorious Quran, from Hadiths and non-Islamic texts, is closest to the Truth, but not absolute Truth like the Pure Holy Quran. This is according to the Glorious Quran Itself:
www.answering-christianity.com/warning.htm
As Muslims, it is
not our problem to sort out the non-Muslims' corrupted scriptures. This is why Prophet Muhammad, peace be upon him, commanded us to neither say they are false nor true when they're used as evidence. We always must say Allah Almighty alone knows what is True and what is false in them. This is also why Allah Almighty Said this about the Glorious Quran:
[018:001] Praise be to God, Who hath sent to His Servant the Book, and hath allowed therein no Crookedness:
[039:028] (It is) a Quran in Arabic, without any crookedness (therein): in order that they may guard against Evil.Only the Holy Quran is without crookedness and man-made lies. All other scriptures are crooked. Again, visit the above link.
Take care,
Osama Abdallah