In any tractate, why isn't the background or explanatory information presented first?
For example, why do the laws of Shechita not begin with the laws of Shechita?
Masechet Chullin (and the Yoreh De'ah) begin the discussion of Shechita by listing those who are approved as shochets. In the gemara, a discussion of the actual details of the slaughter isn't until (unless I missed an earlier reference) daf 27 or so.
In an organized set of laws, wouldn't the foundational aspect -- the definition and the explanation of the topic, be more pressing than a list of who is eligible to perform the action?
In Megillah, the dates of the reading come first. In Brachos, the timing of saying Shma (and not establishing the obligation to say Shma) is first. Both Shabbat and Sukkah assume that one knows that there is an obligation to observe the holiday and the text jumps right in to detailed application of specific rules. [side note -- Kiddushin at least begins with the method but Ketubot, with the timing].
If the goal is to record a set of laws (and as they were oral, they could have been codified and written down in any order) why not set them up in a more logical sequence?