Why weren't the four Hokages reincarnated by Kabuto in the Shinobi war?
Actually, Kabuto can't revive the 1st and the 2nd Hokage simply because Hiruzen Sarutobi (3rd) sealed them away using the Dead Demon Consuming Seal. 

While the reason Orochimaru can revive the two is because he ripped the belly of the Demon (Shiki Fuujin) who consumed their souls in Naruto chapter 618, page 4.