Because the freedom of religion is one of our most fundamental rights not only for Christians. But, it is one of the fundamental rights for all religious people. The attack one religion is an attack on all religions. The attack on the religious freedom of one religion or religious sect is an attack on the religious freedom of all religions and religious sects.
It is not enough for Christians to only defend their own religious freedoms. We need to stand for the religious freedoms of all. We cannot defend our own religious freedoms and be indifferent to the religious freedoms of others. History shows enemies of religious freedom and especially seculars do not stop with infringing upon the religious freedom of one religion or some religions. What starts with a political attack on one religion or some religions branches out to an attack on all religions.
If we do not defend the religious freedom of people of all faiths as Christians, we might not have anybody left to stand with us in the defense of our religious freedom. It works both ways.