The reason for Dill's absence was owning to a new family arrangement. Dill had a new father. Presumably his mother remarried. Dill does not go into details, but he does write of this new development in a letter to Scout. Moreover, Dill stated that this summer he planned to build a fishing boat with his new father. Here is the quote from the book:
But summer came and Dill was not there. 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.
As we can see, in chapter 12, Dill and Scout have a special friendship. Dill really does like Scout a lot. Later on, Scout admits that she had a hard time knowing that Dill would not be with them for the summer. She writes:
The fact that I had a permanent fiancé was little compensation for his absence:
Also what made things harder for Scout was that Atticus had to travel for the next two weeks back and forth to the state legislature, which was called into session.