For the same stupid reason there are Al Qaida cells in the U.S. - STUPID WEAK IMMIGRATION LAWS. And now the U.S. and the United Kingdom are going to suffer until they get rid of those cells that exist in the United Kingdom and the U.S.
They are NOT going to do what it takes to get rid of the terror support cells. After all Mr. Cheney told is to simply expect the next attack as a given.