I wanna say because of his brother to bring him back but something makes me think that he refused to bring him back out of respect...
So imma go with he wanted to have more of an edge over the other villages in terms of battle strength ..
If u think about it aside from the villagers (fodder) all the second generation Kage was pretty much equal at least the main ones
3rd raikage idk if he was Kage during this time lol Mu and Trollkage seem to remeber him think
Mu amazing shinobi in terms of abilities
Trollkage dope shinobi
Tobirama fastest shinobi and water user
Idk who kazekage was
They were pretty much equal in terms of abilities so tobirama wanted a way to win a war just in case lol
Bring back the dead shinobi who fought for u do endless explosions etc
Ur army will never die out