Why didnt the Sage trust his younger son?

[Sage6paths]

Since the Sage chose his younger son as his "Successor", the son should be the 10 tails jinchuriki, no?

A point to note is all the tailed beasts cant seem to give a hoot about the two sons of the Sage.

 

[AllomnisymbolsparttwoT]

rikudo already foretold the ten-tails revival would be when the world ends, the successor's role is to show both men and energy(bijuu) the true path to peace