It takes a ridiculous amount of time, planning and co-ordination to do something like this. You can’t just nip down to Aldi and buy a bootload of explosives and Kalashnikovs, you know. And as for persuading a dozen people to wear suicide vests, another level of complexity.
2. No, they couldn’t have detected that. Full surveillance is difficult, and needs resources.
3. Daesh exists because we created the conditions for it. Thank you, Mr Bush & Mr Blair. It’s easier to make a mess than clear it up. ISIL has clear roots in scriptures, so unless you bomb all the local schools it will grow until superceded or successful.
http://www.theatlantic.com/magazine/archive/2015/03/what-isis-really-wants/384980/