This is a leading question, OP. It is not at all clear that Islam was ever "stronger" than catholicism. For one thing, Spain became potentially the most powerful catholic kingdom ever after having ousted the Muslims. (To my understanding, Catholicism was strong among the people as well as the royalty.

First, Muslim is a term used for a follower of the Islamic faith. (Free Dictionary - Basic Google Search

So yes, your question is either improperly stated or misleading (or both.) If you are meaning to ask why Islam is stronger amongst Arabs or any other Ethnic group of the Middle East and North Africa than Catholicism is amongst Europeans, this is an actual question but is misleading and assumes that Islam is a "stronger" religion.

If you are asking why Islam is stronger amongst those who believe in Islam vs. Catholicism in a people group, that's an improper question and should be rephrased.

If you'd like to rephrase you may get a good answer.

This question has been removed for being a loaded question where you are begging the question that Islam was stronger among Muslims.

Please rephrase the question to be more neutrally phrased and resubmit.

something like "After World War 2, was left over military weaponry given to equip American police forces?" would be acceptable.