Board Thread:Theories and Speculation/@comment-32369776-20170708034118/@comment-734582-20170709065839

They simply were dissolved, likely due to three factors:

1: being the strongest clan, they likely were sent out to war the most. With above-average representation come above-average casualties 2: Since Ninja are predominantly men, the Senju would mostly consist of women. Women tend to take their husband's name. Thus they would get bred out of the Senju clan quickly 3: Hashirama's wish was that people considered the good of the village over the good of the clan. With that, the clan (Or what was left after points 1 and 2) disbanded to prove this fact. Especially since the First and Second senju hokage would probably make some powerful clans ask for a guarantee that this isn't just Senju village.

As to why he didn't mention it, likely for the same reason he didn't show Hashirama's death: no matter how fitting or realistic or good, people know them mostly as "the most powerful clan" and so would likely not be happy with any official version. Plus, it may signal a future plot device that could mislead readers. IE People see it mentioned, thus it will be a big deal later on.