There's not enough information to deduce why Kabuto didn't revive Izuna and therefore, a lot of possibilities. 1. Doesn't Madara have Izuna's eyes? Kabuto would revive a blind Izuna (correct me if im wrong). 2. Maybe like Jiraiya, his corpse was in unreachable depths. 3. Extremely well hidden. Maybe more well hidden than Madara's body. (though imo this is unlikely). 4. Maybe his body rotted away before Kabuto was even born.
I'm sure I can think of more theories but these are the first that come to mind.