Because Bush failed to destroy them.

Guardian:

Al-Qaida has revived, extended its influence, and has the capacity to carry out a spectacular strike similar to the September 11 attacks on America, one of the world's leading security thinktanks warned yesterday.

There is increasing evidence "that 'core' al-Qaida is proving adaptable and resilient, and has retained an ability to plan and coordinate large-scale attacks in the western world despite the attrition it has suffered", said the London-based International Institute for Strategic Studies (IISS). "The threat from Islamist terrorism remains as high as ever, and looks set to get worse," it added.

"The US and its allies have failed to deal a death blow to al-Qaida; the organisation's ideology appears to have taken root to such a degree that it will require decades to eradicate," it continued.