Well, I think most people get their knowledge of Nietzsche, Engels etc from other books that talk about those ideas in a more accessible way. Often this is a distilled version of the argument, made more interesting than the walls of text you speak of. Maybe that's why Hegel and others are used in much more disciplines. They are probably known for their brilliance / influence within academia, but I think that the popularity of any given author beyond academia is highly dependant on adaptations of their work. Thus I think that the interestingness of an author's idea is a factor that has influence on the author's success and maybe a good reason why some are considered legendary in the canon and others not.Frankly, most of the people listed are so fucking boring to read and understand that after 4 years of undergrad work, I was ready to build a time machine so I could go back and edit in some illustrations just to break the castle-walls-of-text monotony.