[andersource]

That's the Pareto principle [0]: 80% of the effects come from roughly 20% of the causes (or effort, in this case).

Very hand-wavy but I think that's also the reason things like expertise exhibit a power law distribution - if as you progress each "unit" of improvement requires 4x the effort of the previous "unit", it makes sense that only about 1/4th of people will make it through. Of course that's very oversimplified but I think you get the gist.

[0] https://en.wikipedia.org/wiki/Pareto_principle