The tractates of the Mishna are in approximately descending order. That is, in each Order, the longest tractates come first and the shortest ones are last (although there are a few exceptions).
In some cases, like Moed, this order makes sense, since there is a natural progression from Shabbat (most common) to the Biblical holidays in order of their occurrence, to the Rabbinical days. However, in other cases such as Nashim, having the tractates ordered by size creates an unusual sequence. Nashim starts with an unusual case, followed by the responsibilities of marriage, then laws tangentially related, another unusual case, divorce, and finally marriage (which ought to come first, as it does in the Rambam and Shulchan Aruch).
So why is having the tractates in descending order so important?