Muhammad Qutb was freed, according to this, because he was a Muslim. From the New York Times:
BAGHDAD, Iraq, July 27 – In the end, Muhammad Mamdouh Qutb figures it was his captors’ remorse that led to his freedom.
Yes, they roughed him up and bundled him into a car, took him hostage for four days and told the world they would kill him. But then, said Mr. Qutb, an Egyptian diplomat, it dawned on them that he was far from an ideal target: he prayed five times a day, he fasted, and as they learned from a television report, he was known for teaching the Koran to children at the neighborhood mosque.
It did not hurt, Mr. Qutb added, that his captors, who called themselves the Lions of Allah Brigade and snatched him as he came out of a local mosque on Thursday night, had accomplished their mission without killing him. Taking him hostage and airing their demands on television was a cheap, efficient way to make their grievances known.
Judging from their accents, he said, he believed that his captors were all Iraqi. Their goal, he gathered, was to impress upon Egypt that the price of aiding the new Iraqi administration would be steep.
“They wanted to send a message to the Egyptian government,” Mr. Qutb said during an interview on Tuesday inside the Egyptian Embassy. “They did that through the media.”