Board Thread:Naruto Discussions/@comment-1512913-20170417105433/@comment-25122992-20170418201844

Either way, Obito got a new half of his body made from Hashi's DNA, who both stemmed from Senju (and thus had their powerful life force) and was an Asura reincarnate (and thus had his chakra). Add this to his Uchiha (though non-Indra's) chakra, and pure logic will allow you to see that he should've been more suitable candidate for the Rinnegan than Nagato who possessed only the life force of Senju (as an Uzumaki) without even having the chakra of Asura himself and of course without Uchiha chakra.

To put it simple, Uchiha chakra + Senju life force + Asura's chakra of Obito > Senju life force of Nagato.