Why? Because "we [Catholic Christians] do not have the same relationship with God." Try telling that to dhimmi Methodist Rev. Susan Boegli, who believes Muslims and Christians "share the same God, the same principles of loving one another, of peace and justice. How we do that, the difference is in the details." She even has a John Esposito DVD affirming this position.
"Vatican cardinal assesses dialogue with Muslims, Hindus, others," from Catholic Culture, January 5:
Asked whether theological dialogue is possible with Muslims, Cardinal Tauran quoted remarks recently made by Pope Benedict and affirmed that theological dialogue, strictly speaking, cannot take place between Christians and the followers of other religions because "we do not have the same relationship with God and much less with our respective sacred books."
Intercultural dialogue is possible, though, as well as about religious topics such "creation, life, family, prayer, fasting, eternal life," and thus, according to Cardinal Tauran, it is possible to speak in a loose sense about theological dialogue. Quoting the Second Vatican Council, Cardinal Tauran added that the Church esteems all that is good and true in other religions and works with the followers of other religions for justice, peace, and charitable initiatives.
Cardinal Tauran added that the sacred books of other religions ought not to be referred to as "Sacred Scriptures." Catholics in formation in "institutes of instruction, seminaries, and novitiates" should study the founders of other religions, while "we Christians obviously have the duty" of introducing the Bible to followers of other religions...