Simple and Straight-Forward
The content of the story is deep, like ocean deep, but it's very simply written. Leigh (and Beverly Cleary) are writing like sixth-graders.
Since it's all letters and diary entries, most of the chapters are short, and there isn't a lot of description that takes forever to read. The sentences are simple and the vocab not difficult, and there's lots of dialogue and action to keep the story moving.
It's also very straight-forward; Leigh doesn't make the reader read between the lines or do a lot of guessing.
For example, when he's starting to be friends with Barry, he says:
I wasn't sure Barry would like to come to our house which is so small compared to his, but he accepted when I invited him. […] That made me happy. It helps to have a friend. (55.2)
Leigh tells us exactly how things are: he wasn't sure, Barry came, he's happy. We know where Barry and Leigh stand on house and friendship issues, and there's no guesswork involved.
It's nice when a story can be deep like the ocean but easy to get across, like we're parasailing or skiing over it.