That very question has kept me up at night many times. I mean they had his body and everything!!!!It's weird, if Nagato was able to revive people from times ago, he could have revived Yahiko as well. ö_ö
Maybe they realized he was more powerful as a martyr than he would have been if they brought him back to life. If that in death he created a movement that took them toward peace that his resurrection would have defeated.That very question has kept me up at night many times. I mean they had his body and everything!!!!