Dill does not come for the summer because his mother remarried.
When summer comes at the beginning of Part Two, Scout contemplates how her “permanent fiancé” Dill does not come.
I received a letter and a snapshot from him. The letter said he had a new father whose picture was enclosed, and he would have to stay in Meridian because they planned to build a fishing boat. (ch 12)
Dill does send a picture, and his new father has “pleasant face.” Scout is upset because Dill said he would marry her, and then he does not come. Scout enjoys the summers with Dill, and summer will not be summer without him. Asking Scout to marry him is just his child's way of trying to get himself a family.
Dill does show up, and Scout finds him hiding under her bed. He has a story about how his new father chained him in the basement and he escaped and took the train, but in reality he just felt that no one needed him.
"Well, they stayed gone all the time, and when they were home, even, they'd get off in a room by themselves." (ch 14)
Dill comments that his new father did not build the boat like he promised, and his parents just sat in a room alone reading, and didn’t want him with them. Lonely, he had headed to Maycomb where some people did care.
Dill’s escape demonstrates how the Finches are more of family for him than his own. His mother has no time for him, and his new father makes promises he can’t keep. Dill lives for summer, the only time when he feels a sense of belonging.