Quote Originally Posted by Yukimura
I believe it was also stated that not all Uchiha obtain the Sharingan, though that could mean not all are able to eventually activate it but all are born with the potential or it could mean some people been of two Uchiha parents just won't be able to activate it at all and some will. I don't think we've been given enough info to be sure of either.
Well in the case of the Uchiha from what we saw during Sasuke's flashback of when Itachi killed the clan the majority of them weren't even ninjas and thus couldn't hope to activate the sharingan. I think why most of them don't get it is simply because it requires special circumstances to be activated.