Simple; Symbolic
Bunyan intended this story to be easily accessible and digestible, and his writing style reflects this. The sentences are easy to understand, especially when the message is extra important, like this:
For God can love him, and forgive his sin. (P110)
Yup. Hard to get any more straightforward than that bad boy.
The fact that the whole story is an allegory, however, means that every simple sentence, every description and line of dialogue is pointing toward something more abstract and profound.
Saying, for example, that Christian was trapped in Doubting Castle until Hopeful helps him remember that the key, Promise, will set him free is a lot more loaded than saying that John was trapped in the shed until Billy reminded him of the key next to the watering can. Or, for a more gruesome example, check out Faithful's torture scene:
They therefore brought him out, to do with him according to their law; and first they scourged him, then they buffeted him, then they lanced his flesh with knives; after that, they stoned him with stones, then pricked him with their swords; and last of all, they burned him to ashes at the stake. Thus came Faithful to his end. (P500)
Your first response is probably "Yowch." Your second, if Bunyan is doing his job, is to think of a few things. One important thing that this paragraph is pointing toward is the crucifixion of Christ: he was tortured at length before dying. This suffering is integral to understanding Christ's love for mankind, and Bunyan wants you to think about this.
These tortures also symbolize the hardships of living a good Christian life. Being stoned, especially, harkens back to the idea of judgement—historically (as well as in the Bible), people who were stoned were often being punished for going against cultural norms. So Bunyan is telling the reader that Christians will be up against a whole lot of judgy nonsense in this world, and a lot of it will hurt.
Pretty much every action and detail in this book demonstrates and defines a larger concept. Finding and constructing these meanings in your own head can take a bit of patience, but, well, that's probably just what the Rev. Bunyan had in mind for you.