The American FBI has found, as it expands its informant network in the American Moslem community, that many of the more troublesome members of mosques are recent converts, who were radicalized by what they found on the Internet. These converts, as is the case in many religions, tend to be more energetic in the practice of their newly adopted faith. Many are also looking for a cause, something that will enable them to excel at anything, even at the risk of their own lives. This causes problems if the goal becomes Islamic terrorism. As a result, mosques are becoming more careful about accepting new candidates for conversion. Those that reek of fanaticism, or show too much enthusiasm for "defending Islam," are told to go elsewhere.
It used to be that there were plenty of mosques in the West run by radicalized, or very conservative, clerics. But no more. Since September 11, 2001, Western nations have cracked down. Radical clerics were either expelled from the country (these guys tend to be migrants), or police investigations of criminal activity by these firebrand clerics put them in jail, or on the run. Turns out that in the 1980s and 90s, a lot of mosques had been taken over by a small number of radical members. Threats and violence were used, and it often got quite ugly. This was ignored until the 1990s, and not tolerated much at all after 2001. While this kept the radical Moslems quiet, it did not always change their attitudes. But these men were usually migrants, often from the same country, or region, as their fellow congregants. After 2001, Moslems in the West, particularly the United States, knew that they were responsible for the terrorist activity of Moslems they worshiped together with. They could usually recognize another migrant who might be up to something dangerous. Action could be taken to stop it, or drive the troublemaker away (or turn him in to the cops). But the new converts were harder to read. Thus the growing tendency to scrutinize those seeking instruction to become a convert.
Even before September 11, 2001, al Qaeda warned its agents to not hang out with American Moslems, who were known to turn in suspected radicals. Now American Moslems are letting the FBI know about suspicious new guys seeking to convert. That has led to the arrest of some active terrorists. But in many European countries, local Moslems are less prone to let the police know of someone suspicious, and there are more active Islamic radicals (because the Middle East is closer, and Moslem migrants are less likely to be accepted by the natives.)