>>508629958 (OP)Alcoholism and the decline of the Catholic Church.
The church provided the structure necessary to keep society from degenerating, but once the reformation happened too many people had too many weird ideas and they spread, slowly corrupting the morals of the elite upper class, who than passed their degeneracy down to the masses.
Alcohol was purified by industrial processes during the late 1700s to become a lot stronger and thus have far more devastating effects on peoples health and judgement. A population with increasingly impaired judgement and a lack of structure and things degenerated fast.
For these reasons the Muslim world hasn't degenerated.