Why all Al-Qaida bomb plots, actual or planned, involve undercover western agents or western converts to Islam helping with material and logistics ?

First and foremost: a famous convert.

The latest Illinois and Texas bomb plots (2009).

The 2006 London bomb plot.

The 2006 Toronto bomb plot.

The 2005 London bombing.

The 2004 Madrid Bombing

A German convert to Islam describes meeting 911 plotters.

Adam Gadham Pearlman, a Jewish convert to islam behind Bin Laden videos