Over seven years Iraq had 1003 suicide bomb attacks. That's far from zero, but hardly and endless tide. When you consider 9/11 where preceded by an early 'failed' attempt: https://en.wikipedia.org/wiki/Bojinka_plot Which included a test bomb that killed one person but failed to down the aircraft. And out of 26 people that attempted 9/11 only 19 where part of the attack you find they actually got somewhat lucky even before heightened security.

The absolute threat is just not that high.