What I would like to know is why it is limited to theologians and not open to philosophers?
Theology and philosophy go hand in hand. It is part of the intellectual renewal to ground theologians in the history of philosophy, its methodologies, and metaphysics. Otherwise you run into problems of credibility - not within the eyes of the world, but within the rigors of the field itself.
Philosophy has had a huge impact on the history of the world and our understanding of being - to be ignorant of that history is to deprive yourself of insights and leaves you open to errors that have been repeated time and again. The methodology of logic is indispensable for any being aspiring to the study of higher things. How do you politely say "oh hai!!! ur doing it wrong, Mr. Theologian - middle term must be distributed at least once in t'eh premises." Reason must be sharpened. And finally - metaphysics. How to stress its importance enough? I think I'll just point here.