I began questioning the commonality of SJ types, especially ISTJ and ISFJ, very early on in my study of type theory (I do still contend that ESTJ and ESFJ are somewhat common, though I think they are fewer in number compared to some E__P types, especially ENTP and ESFP).
Let's consider the basic contentions of S and J: they are regulated, controlled, decisive, respectful of tradition, aware of their environment, aware of common sense, and aware of their appearance.
Now, let's consider what we see at the average school and the average mall and the average Walmart as we keep in mind Voltaire's brilliant paraphrased quote, "Common sense is not that common."
Most of the teenagers I teach, most of the people I see at the mall, and most of the people I see at the average Walmart, are like this:
* don't give too much of a crap about school or following the rules
* wear in-your-face t-shirts that reflect their favorite popular culture icons
* don't seem to notice how disheveled and goofy they look
* can't follow rules that well
* hate reflecting or writing about much of anything
* see global truths, but not any introverted or detailed depth
* daydream and spazz out constantly
* don't notice much of anything about their environment
* have an indifferent or hostile attitude toward tradition
* can't sit still to save their lives
I mean, seriously? Are we so wrapped up in our imaginative heads that we don't notice these things when we should? Now that I have stopped trying to daydream as much as possible wherever I go, reality has finally reached me. SJs are NOT everywhere. Common sense is NOT common - Voltaire was right. Most kids are NOT proficient in Si. Most kids do NOT respect authority very much, and neither do adults, though maturity helps this to an extent (types balance more as they get older).
I think ENTP and ESFP are the most common types in our nation right now. We want our freedom, we want our fun, we want our excitement, we want our instant gratification, and we want escape from reality - few people NATURALLY want to maintain boring tradition, become accountants, clean their homes, or follow and plod over every detail in school (or even at work!) unless FORCED by their job requirements.