George Saliba, author of "Islamic Science and the Making of the European Renaissance" discusses the cause, and more specifically the timing, of Islam's scientific and cultural 'decline'.

TL;DW? He points to the continued scientific output of Islamic nations up until the 16th century, and argues that the question shouldn't be "what went wrong with Islam" but: "what went right with Europe" around that time? Instead of trying to link an objective scientific decline to the Mongolian invasion, the Christian Crusades, or the teachings of al-Ghazali, which all came before the decline, we should instead acknowledge that every culture at the time experienced a huge relative decline compared with the extraordinary growth and development of Europe. At the time, Europe experienced unprecedented growth in wealth, trade, culture and territory, and this engine catapulted its scientific leadership to a position that the western world maintains today.

While some of his specific points can be debated, the overall picture seems far more robust than pinning the decline on a single individual, which non-historians like Neil DeGrasse Tyson or Salman Rushdie have attempted to do.