Why can't Dill make it for the summer? Why is Scout especially disappointed?

Dill usually spends the summer holidays in Maycomb. It's a lot of fun for Jem and Scout to hang out with him, as he's always full of great ideas for playing games and getting up to mischief. But one summer, Dill writes to Scout, telling her he can't make it, and Scout's very upset at the news. To make matters worse, Jem's been growing up quickly, and behaving in a stand-offish manner toward his little sister. He increasingly wants to be alone, and no longer has any time for what he considers Scout's and Dill's childish games. Scout already feels lonely enough as it is, but Dill's letter makes a bad situation worse.
In his letter, Dill explains that he now has a new dad, a lawyer, and he's going to spend time with him over the summer. Dill and his new stepdad are going to build a fishing boat together. Scout's happy for her friend, but still pretty miserable at the prospect of a long, hot summer without him. Dill's a tough act to follow, and for Scout there are no other children in Maycomb available to fill the huge void he's left behind.