You’re assuming leftism isn’t in large part a genetic disorder.
It definitely is genetic, and it's not related to just the mental aspects. It's also has a physical component, to the extent that it reverses the long held question of "why are conservatives attractive"? The actual observation should be, "why are attractive people conservative"?
The answer to this is related to studies and analyses on historical human breeding rates. As it turns out - logically - before welfare, the percentage of men & women out of the entire population who either were married and/or had children was lower than today.
In fact, some researchers conclude it was on the order of 80% for women and 60% for men. In other words, most people had the economic means, therefor adequate mental and physical qualities to engage in social commerce sufficient to be able to participate in family formation & support.
Once the basic economic requirement of self-sufficiency was eliminated, then the bottom of the genetic pool was enable to breed without restraint. The results are what we see today with a veritable circus freak show exhibit of all kinds of weird, recessive traits that has previously long been held in check.
I’m sure the leftism disease can be both by nature and nurture.