What makes 20th century modern poetry "difficult" and what are some different types of difficulty?

i have read the novel roll of thunder hear my cry by mildred d. taylor.

1. this isn't like any other story i've read or watched because it is about a black family in mississippi right after slavery in the u.s. had become illegal. most of the novels i have read were about slavery. another point is that the author of the novel used symbolism in a way i haven't seen before. for example she uses thunder as something negative meaning something bad will happen. but by the end of the novel, she made thunder a good thing.

2.   one character that changed during the novel was stacey. stacey is the eldest child of the black logan family. around the begging of the novel stacey was avoiding the white boy jeremy simms. he had a dislike for jeremy's family. but at the end of the novel stacey began to like jeremy.

3. the author starts the novel with the problem that the children of the logan family have to avoid the white kid school bus while walking to school.

the two sonnets of sidney most closely follows the division between the octave and the sestet are the astrophel and stella sonnets.