Newton’s Principia is totally and absolutely impossible to understand to the average curious person raised on Cauchy and Weierstrass. There was a course at university aimed at people with advanced math knowledge to prepare you to read it.
Popularity is more a question of marketability, especially in the dire non-fiction category. Reading is a secondary use for most of these books, and it makes it even harder to discern which lifestyle books are resume-padding filler or truly generalizable advice.
Possibly Mein Kampf could compete in short passages by a "reading difficulty" score, but it's really tedious and much longer (by a factor of 5!). Darwin could actually write. Origin of Species was intended for the educated public as well as for scientists.
Hold on, hold on.
I started to read Origin of Species.
One time. But I did read it. Not all. Unconvincing.
And I did read all of Gabriel Marcel's Metaphysical Journal. So ... not a matter of Slack. If one in 100 reads Origin that's a rough approximation to the number of masochists