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I still don't know for sure, but maybe because he was convicted in absentia in Egypt, where convictions in absentia had been going on for some time. Many defendents in the Luxor murders were evidently convicted in absentia in trials like the one below during the 1990s, which I found on a search from Egyptian trial data. These trials were later ended due to complaints and pressure from human rights groups (no doubt many of them now known to finance terrorism as well as the more legitimate international groups) :

"The last major military trial of this kind was held between February and April 1999, involving 107 defendants, 60 in absentia. Most were charged with membership of an illegal organization (al-Gihad), as well as other charges including conspiracy to commit murder, weapons possession, and forging official documents. Several had been arrested the previous year in Albania, reportedly in connection with a planned attack on U.S. interests there. Several other defendants had been extradited from Bulgaria, Azerbaijan, Saudi Arabia, the United Arab Emirates, and Libya. The Supreme Military Court sentenced nine to death, among them Ayman al-Zawahiri in absentia. Seventy-eight were sent to prison and twenty were acquitted. As in previous cases, a number of defendants stated in court that they had been tortured. In July 1997 five members of al-Gama'a al-Islamiyyas leadership issued a statement from Tora Prison, where they are currently serving prison terms, calling for a halt to violence. With the exception of the killings at Luxor, this directive has been largely adhered to and has been reiterated on several occasions since. As a result, several thousand Islamist detainees have been released since 1998, most of whom had been held without trial."