How reassuring, not (part 1)

This Blaze article http://www.theblaze.com/stories/2016/04/11/german-intelligence-chief-makes-surprising-admission-analysts-didnt-expect-terrorists-to-sneak-into-europe-with-migrants/ contains two not very reassuring comments.  The first is the headline revelation:

'The head of Germany's domestic intelligence agency said in an interview published Sunday that security officials had misjudged the Islamic State group's ambition to sneak terrorists into Germany under the guise of asylum seekers.

In an interview with the German newspaper Welt am Sonntag, Hans-Georg Maassen, head of Germany's Federal Office for the Protection of the Constitution, or BfV, admitted there had been a "misjudgment" regarding the Islamist group's strategy.'

Why would a European intelligence agency think that ISIS wouldn't try to sneak terrorists into Europe amongst the 'refugees' aka economic migrants aka invaders? After all it's not as though UKIP politicians and others hadn't been warning of this.