Board Thread:Theories and Speculation/@comment-26904721-20150820181613/@comment-26773508-20150821130202

Thekillman wrote: Considering that the Senju were both under Hashirama and Tobirama (who believed in the Village above a clan), and that they were the strongest ninja alongside the Uchiha, the answer is likely that they died out through battle and marriage. Unlike the Uchiha and Hyuga clans, who specifically upheld their Clan status, many other clans just integrated. Hence, the Senju clan likely got dissolved. If the Senju were the strongest, which they were, then they were the least likely to die in battle, unless a concerted effort was put by many collaborating clans/nations to wipe them out, as happened to the Uzumaki clan and their village. But as of now, there is no evidence to support that thesis. So I think the most likely explanation is that they intermingled with other clans.