The answer of course, is that the many thousands of man hours it would have required to put under surveillance everyone who came into contact with these 5 men was just too much for our intelligence services.
Whose fault is this?
Well the answer is, it is no-ones fault. Yes, there was an intelligence failure and any lessons that can be learned need to be learned. But the government cannot see into the future any more than the electorate. Where was the overwhelming public call for more resources for the intelligence services? The answer is, there wasn't one. In fact there probably would have been an outcry against such a thing from some quarters.
If only those who blame the government over this would put as much effort into campaigning for better funding for health, education, public transport and reducing inequality - things that would improve and save far more lives.