...is it simply that it is explained better in later chapters, so just memorize for now and understand later?
How far along are you? I recently finished the Rita book, and felt that the order as well as the impact of the whole Process Chart is hammered on throughout. It leads off every chapter and many of the exercises deal specifically with the process chart and relating that particular chapters info to the process chart.
Put another way, it didn't make a lot of sense to me when it was introduced, but is much more useful now that I've made it all the way through and have a better perspective of the whole.
Does anyone know where I can find more explanation on the reasoning behind the sequence?
Unfortunately, the correct answer here (as it is for most PMP specific questions) is "read the PMBOK". I did all the practice exams at the end of each chapter and referenced the PMBOK for questions that needed more explanation and the order makes sense now. So somewhere in the combination of reading the Rita book, doing practice questions, and reading the PMBOK your clarification and reasoning lies.