Board Thread:Theories and Speculation/@comment-995176-20161007184843/@comment-25632268-20161007192515

Well, we don't really know if it never happened, as far as manga is concerned, Senju clan just suddenly disappeared, we don't even know if Tsunade's last name is Senju as I think gender of Hashirama and Mito's child was never mentioned. Other than that Senju clan either does not exists by the time Naruto is born or is scattered and insignificant.

And about Hashirama and Madara - well, Madara left before father or mother of Tsunade has been born (it was mentioned during Kushina's flashback that Mito's seal nearly broke when she was giving birth, meaning she got pregnant after becoming Kurama's jinchuuriki and that happened after Hashirama vs Madara fight). Besides, Madara was so centered around himself I doubt he would even consider thing like marriage between clans.

Other than that, reasons are probably the same why Hyuga and Uchiha never arranged it or Hyuga and Nara or other clans - because Kishi never thought about it, it was unimportant for the story.