I don't think I agree that it's entirely random. The solid majority of people in my family (myself and one of my nieces excepted) are Ns. There's a broad range in terms of education and interest, but from what I've heard from others, this isn't unusual - many people seem to come from families that are disproportionately of one temperament (my family is evenly split between NT and NF, with one SP and one SJ).
Also, I don't know how it's possible to *know* that the majority has always been and will always be SJ. Just one measurement: If SJs populate most civic groups, and membership in all groups has dropped drastically over the past 40 years, isn't it possible that SJs are less common than they used to be? Same with other obvious bastions of the SJ - mainline religion, major political partys, neighborhood associations... all down. People are less likely to stay in the same job for their entire career, and more likely to move away from their hometown. I know there are other contributing factors for each of these trends, but it does seem that taken as a whole, maybe there just fewer SJs out there.