What factors led to Grayson's inability to read and to his running away in Maniac Magee?

Grayson's difficulties started when he was very young.
He didn't do well in school and was put in a class that did not teach anything, but instead the students just did simple art projects. The teacher even said she didn't believe the kids would learn anything. Grayson decided to stop trying and probably never learned anything of consequence scholastically since—until Maniac comes along.
Grayson tells Maniac these stories while they are living in the Bandshell. He also tells Maniac about his alcoholic parents, who contributed to his running away at age 15 to join the Minors.
Grayson probably appreciates Maniac's rapt attention and lack of judgement, while at the same time pushes him to learn to read. Maniac is basically homeless, other than the Bandshell, and Grayson can relate his journey as both an adventure (striking out Willy Mays) and a warning (homelessness and poverty).