Why wasn't Dill able to come to Maycomb that summer?
To Scout, Dill meant summertime. She looked forward to him being there to play with, and to be her fiancee'. On page 117, Scout narrates to the reading audience:
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. His father was a lawyer like Atticus, only much younger. Dill’s new father had a pleasant face, which made me glad Dill had captured him, but I was crushed. Dill concluded by saying he would love me forever and not to worry, he would come get me and marry me as soon as he got enough money together, so please write.
Remember, Scout is a narrator that is very young and Dill is a kid that lies. These contents of his letter make Scout feel like there are reasons that he isn't coming. From what we as readers know about him, we can see that he moves from relative to relative, so we can't always believe what we hear.