The reason Izumi cannot be revived.
When people die during missions they are sent to Gantz's memory and can be revived with 100 points, but Izumi was never transported to the memory and therefore cant be revived.
How do i know this?
because it was on the news the next day that his body was amongst the dead people who were killed the night before. Its obvious that when people are sent to the memory, their body's are transported, like how what happened to tae, but izumi's body was not teleported to the memory (cause his body was found), so izumi cant be revived with 100 points because he isnt in the memory.
tae like all other targets, are transported at the end of the mission, i know this because their alien corpses were not left on the street for people to find, or else everyone would know about aliens cause they found the bodys.
My theory is that all of the targets corpses and fallen gantz members corpses are transported after the mission is over, humans go to the memory, wich would explain why tae went to the memory, and aliens go to an unkown location. Aliens and gantz members who die outside of missions are not transported, for example the Shorty Alien that killed all of kuronos classmates. that aliens body dint go anywhere when kurono killed it, it was taken by the police (remember kurono said that government agents went to his house and told him to keep his mouth shut about wat happened and the alien) unlike the aliens/targets who die during missions who's body's are never found because they are transported like tae.
Some of you may say that the reason why they cant see the aliens is because they are on another frequency, but they dont stay that way after the mission, because if that was the case then the shorty alien that killed kurono's classmates would have still been invisible to humans after the mission, but that was not the case, humans(izumi who wasnt a gantz member at the time and police) could see the shorty alien. even if the frequency thing was true now that aliens are visible all the body's of past dead aliens would be visible and everyone would know of aliens, but that isnt the case. on the news the next day after the oni mission when they were talking about how people witnessed the alien fight, they had no alien body's now did they, no. The only explanation is that they were sent somewhere.
Izumi did not die during a mission, and his body was not transfered to the memory (obviosly because the it was on the news the next day that he died, as in they found the body).
Since Izumi is not in the memory, he cant be revived, Izumi is gone for good.