Reasons
Tobirama looked in awe as he looked at the the wooden structure that he had created. It looked somewhat like a tree? It looked twisted and mangled however. How that was even possible is beyond him. But that didn't matter.
The experiment was definitely a success.
He definitely needed to keep this a secret.
This could turn the tide of battle, a power that his enemies never expected him to have. Tobirama also needed to master it. He only had so much time to do things though....
But that's what shadow clones were for.
Or at least, one of the reasons.
Assisting his brother.
Experimenting.
Training.
Tobirama usually assisted his brother in person, not a shadow clone... except for when his brother wasn't around and he was just listening to others on his brother's behalf--- that's something he uses a shadow clone for.
But really, his life was falling into a routine of doing three different things, in varying order. Sometimes overlapping with each other with the use of shadow clones.
Hiruzen and the rest of his students had long been promoted to chunin and were mostly independent from him now. So that freed up more of Tobirama's time since he was no longer teaching.
The number reasons he stayed in the village had detiorated over time.
The woman he loved? Dead
His friend Tatsuma Uchiha? Dead
His friend Madara Uchiha? Dead
Hashirama Senju? Still alive and well for now.
Tobirama hoped Hashirama would stay that way.
It was foolish for him to keep his hopes up. Very foolish. Tobirama should have known better.
Tobirama didn't want to believe the news. He didn't want to believe what he was told.
Hashirama Senju was killed in battle.
Hashirama was killed.
His brother was dead.
Hashirama was d e a d.
What was the point? What reason had he to stay in the village now? What reason was there for him to live?
Oh.
He was Hokage now. Tobirama didn't want to be. He wanted to throw himself into science and working on his experiments full time. He wasn't suited to be Hokage. Being an advisor? Yes, he could do that, but being Hokage required more than intelligence. It required more social skills and diplomatic tact than what he possessed.
Tobirama was also was the head of the Senju Clan now. He definitely hated dealing with matters regarding his clan. Especially after how the elders disapproved of Her and the rest of the situation that arose from that.
This wouldn't last long.
He needed to leave.