People often, but not always, lack the prerequisites because they weren't able to learn them when it was taught in school. And that was because they lacked the pre-pre-requisites because they in turn didn't pick those up when that was the material being taught.

In math, things build upon previous things to much greater degree than in other subjects. If you get off track once, it's hard to catch up.

But if you lack prerequisites because it was never taught in high svhool etc, that's a failure of the curriculum.