Every single one of Indra's reincarnations were Uchiah, so why is Ashuras different with Senju and Uzumaki both being reincarnations: Hashirama Senju, Naruto Uzumaki.
The Uzumaki and Senju both share blood ties. Indra and Ashura's chakra don't necessarily have to reincarnate into Uchiha or Senju/Uzumaki;it is just a more likely outcome.
Because it's not explained within the manga, when it's ironically clearly explained that Uchiha are the thinned blood line of Indra and Senju the thinned Bloodline of Ashura. It's only logical for them to be reincarnated - as far as the manga logic of rebirth goes - within their respective clans.
Ashura's reincarnation within Naruto ... makes zero sense.