The question is, Why the Law? A more vital question to address is, What is the Law? First, the Law has been described as an entire tapestry of God's will and divine intent/purpose. No law stands in isolation, each is part of the whole. When one starts picking and choosing pieces of the Law to follow (or not follow), this disobedience results is a lack of discipline which results in an unraveling of the tapestry of God's will and divine purpose for us as a person, as a people, as a nation.
I cannot speak for Jews, but I can imagine that a Christian dismissal of the Law might well be seen as Christians dismissing God's plan and divine intent. To a faithful Jew, one can understand why that would be both blasphemy and heresy.