More rambly and unsubstantiated than usual, apologies —
Theory: Mathematical exposition has evolved just like fiction writing has. However, tradition has held stronger in mathematics (likely due to a need for precision) and it means that clarity in writing is if not actively discouraged at least passively devalued.
Theory: We are not anywhere near the threshold of simplest and clearest explanations in the exposition of mathematical subjects.
Still, what used to be difficult is now considered easier. Various subjects have shifted their supposed level. For instance, not long ago College Algebra was the prestige class at the top of the high school level.
Furthermore, our raw definitions of what each class is has shuffled the actual content of subjects; Algebra I from the 1940s is not the Algebra I of today.
Theory: It would be possible to take a “hard” subject like group theory or transcendental number theory and make it comprehensible at a lower level. However, as there is no requirement to do so there is little motivation to make the subject easier. When a curriculum shift happens to move topics to a lower level, mathematical exposition evolves to catch up.