The very simple answer to the question in the headline is that theology is not science. It is a just study of the basic principles of a religion and the study of its ‘holy’ writings. It has, in fact, very little in common with the normal scientific study of history. This is true, even if modern theology can well use methodology and terminology that are borrowed from the world of real science.
The biggest difference between theology and science is that some of the basic claims in Christian theology, such as the role of religion as a source of morality or the position of the Bible as divine revelation, have to be accepted as unmovable and unchanging basic premises. It is simply impossible to think that a theologian could work in an official capacity, if he would reject the Bible and claim that the real source of wisdom is in the Koran.

In real science, there simply cannot be such immovable and unchangeable basic assumptions. In science one must be able to test and also reject all claims that are being made, if the need does arise. Of course, there exists also the scientific study of religions. However, it is not carried out in the theology departments. It is carried out in a separate field of religious studies that is not connected to any kind of religious ideologies.
In the real scientific study of religions also the Bible can be studied as a historical document which does give light to the world where its writers did live and of their hopes and expectations. On the other hand, in stark contrast to religious studies theology basically just tries to find suitable questions for answers that are already known. It is the exact opposite to how science does work.

Wikipedia

Scientific study of religions uses scientific methods to study all religions. It does also study Christianity, but Christian theology is looking for new justification for holding up this religious ideology and it also trains workers for the religious organizations.
If Christianity should have withered away some time ago, it is quite certain that theology would not be studied anymore in our universities. However, the scientific study of religions would go on as it is now.

On the other hand, if a Marxist world-revolution would have taken place, we would have a scientific study of Marxist dogmas in our universities. This field of study just could be extremely similar to the modern theology, even if we know now that Marxism is not science, but just an ideology. Of course, people study and learn many things in the field of theology also that are quite scientific in their outlook.
However, the basic reason for the very existence of theology is always an ideological one, not scientific. We have no other discipline of science that would have been founded to study and most of all support just one form of ideology, which happens to be a religion in this case.

Theology was born from the need to train new workers for the religious organizations that are forwarding the Christian religious ideology. This fact is not changed by the fact that theology has adopted many of the scientific methods. Its ideological nature has not been changed, as these scientific methods are not used in an objective way that they are used in science.
Admittedly, there is a lot of real scientific study that is done in the theology-departments throughout the world, the more so, as the religious organizations also want real and reliable information of the impact of their work and how the world that surrounds them does develop. However, deep down the whole discipline of theology does exist only to serve one religious ideology and its needs. Because of this theology is not real science.

(This piece was completely refurbished on 31st of March, 2013)