One reason the crusades failed to conquer the holy land permanently was because

The fighting was far away from Europe. Crusades was a series of religious war. It took place in the Medieval period with the sanction of Latin Church. The main war was to capture Holy Land from Muslims, but failed at last.

One reason the crusades failed to conquer the holy land permanently was because the fighting was a long distance away from Europe, but in the same way they had great consequences for them, they demonstrated the religious unity and the power of the church, as well as the expansion of Christianity.