Why the Collapse of Philosophical Studies in the Islamic World?

Leo Strauss sketches an answer in his "How to Begin to Study Medieval Philosophy" in The Rebirth of Classical Political Rationalism, ed. T. L. Pangle, University of Chicago Press, 1989, pp. 221-222, bolding added:

For the Jew and the Moslem, religion is primarily not, as it is for the Christian, a faith formulated in dogmas, but a law, a code of divine origin. Accordingly, the religious science, the sacra doctrina, is not dogmatic theology, theologia revelata, but the science of the law, halaka or fiqh. The science of the law, thus understood has much less in common with philosophy than has dogmatic theology. Hence the status of philosophy is, as a matter of principle, much more precarious in the Islamic-Jewish world than it is in the Christian world. No one could become a competent Christian theologian without having studied at least a substantial part of philosophy; philosophy was an integral part of the officially authorized and even required training. On the other hand, one could become an absolutely competent halakist or faqih without having the slightest knowledge of  philosophy. This fundamental difference doubtless explains the possibility of the later complete collapse of philosophical studies in the Islamic world, a collapse which has no parallel in the West in spite of Luther.

I like the "in spite of Luther."