Israel desperately wants the monks and nuns to leave so they can eliminate the armed Palestinians. No building, no matter how historical, is worth people sacraficing their life over. The priests should just leave and let the chruch be stormed.
Of course, the truth of the matter is the preists couldn't leave even if they wanted to. They are being held as hostages.
You are wrong in asserting that they are being held hostage. They are not hostages, they are fulfilling their calling. You are correct that no building is worth the taking of life, and in the interest of saving lives the priests, nuns, monks, and brothers remain. Please see
this link with information from the Catholic Chancellor of Bethlehem.