The Boston Globe explains Why terrorism flourishes in Brussels. I thought this was a very good introduction to the specific problems as opposed to all the people who'd never heard of Molenbeek yesterday spouting off today as if they're experts.
Still, Molenbeek is a place apart, its largely Turkish and Moroccan population poorly integrated into mainstream Belgian society. Making room for immigrants can be tricky in any society, but in Belgium, there are some additional hurdles. For instance, some jobs require fluency in both established languages — French and Flemish — which is a demanding request for any immigrant.
Over time, disaffection in Molenbeek has been stoked by radical Islam, brought to Belgium in the 1970s by a group of Saudi-trained imams. Their literalist vision was quite different from the brand of Islam practiced by Belgium’s North African immigrants, but over time the more radical figures have built a following.
Even as radicalism flourished, Belgian intelligence agencies have repeatedly failed to prevent terrorist plots. Again, this is where Belgium’s political and linguistic divisions seem to make a big difference. Belgium is a federal state, split between a French-speaking south and a Flemish-speaking north. The central government is actually fairly weak, leaving a lot of power in the hands of local governments — which makes coordinated intelligence-gathering particularly difficult and complicates efforts to amass nationwide resources for an effective, antiterrorist crackdown.