The people who became jinchuurikis were usually related to the kage...
Eg: Naruto (fourths son), Gaara (kazekage's son), Killer Bee (Ay's Brother), Mito (Hashirama's wife), etc.
The Uchiha weren't related to anyone who was a kage because none of the Uchiha ever became Hokage...
Anyway, it's best for the jinchuuriki to be related to some type of Kage in order to protect them and make the villagers somewhat accept them (which didnt turn out the way planned).
If Uchiha became jinchuuriki they'd be pretty OP ..just saying.
