It really makes no sense to me why he wasn't (or didn't use it) for that matter. Uchiha were inherit eye dojutsu weilders (though some didn't unlock) Senju were inherit really proficient SM users (though it wasn't enough Senju shown to support all weren't natural at SM)
Tobirama was a genius, and the best water style user of his time. Mix that with Sage Mode and you would very well have yourself a very formidable foe. Water avatar similar to Susanoo or Wood Budda, FTG, powered by first class intellect. Maybe Kishi and the creators thought by giving him SM, that it would ruin where they were going with Hashirama as the strongest Senju, to me, nah.
Tobirama left the war arc leaving alot of mysteries and unanswered questions behind, overall I felt like I wanted more out of him being a Senju.
Tobirama was a genius, and the best water style user of his time. Mix that with Sage Mode and you would very well have yourself a very formidable foe. Water avatar similar to Susanoo or Wood Budda, FTG, powered by first class intellect. Maybe Kishi and the creators thought by giving him SM, that it would ruin where they were going with Hashirama as the strongest Senju, to me, nah.
Tobirama left the war arc leaving alot of mysteries and unanswered questions behind, overall I felt like I wanted more out of him being a Senju.