Same reason it is difficult to identify when and where local radicalisation happens - it is so entrenched, that it is washed into normal aspects of many everyday activities.
Do you deny that radicalisation is happening at the pulpit and more informally during backyard events and normal private round tables?
Look at Hizb ut Tahrir - they are allowed to openly operate and meet here, despite their clearly damaging philiosophies.
The money trail beginning with seemingly harmless Halal taxes and ending who knows where, via whatever channels would be too difficult to trace.
