The 39 melakhot and the Mishkan

On Shabbat 49b, we are told that three early amora'im, Rabbi Yonatan ben Akhinai, Rabbi Yonatan ben Elazar, and Rabbi Hanina bar Hama, were sitting discussing a very particular case of muktzeh that came up in the mishnah. Then, in classic Talmudic fashion, we are told as an aside that those amora'im also discussed the most fundamental question of our massekhet:

הדור יתבי וקמיבעיא להו: הא דתנן אבות מלאכות ארבעים חסר אחת כנגד מי?

אמר להו רבי חנינא בר חמא: כנגד עבודות המשכן.

אמר להו רבי יונתן ברבי אלעזר כך אמר רבי שמעון ברבי יוסי בן לקוניא: כנגד מלאכה מלאכתו ומלאכת שבתורה - ארבעים חסר אחת.

Why are there thirty-nine categories of forbidden work on Shabbat? What do they represent?

Tractate Shabbat: In and out and in and out...

Massekhet Shabbat discusses, of course, the laws of Shabbat. The beginning of the massekhet is structured chronologically, starting with the laws of Friday afternoon, then candle lighting on Friday evening, then cooking and insulating Friday night. The basics of the 39 melakhot don't come up until the middle of the seventh perek. The rest of the 24 perakim then flow from topic to topic.

But amidst this structure, a surprising topic dominates the first half of the massekhet: hotsa'ah, the melakhah of carrying between a private and public domain. Eight out of the first eleven chapters of the massekhet contain mishnayot about hotsa'ah, and it is the sole topic of five of those chapters: 5, 6, 8, 9, and 11. And most strangely, the very first mishnah of the massekhet is a total non sequitur to the rest of the first perek—a mishnah about, you guessed it, hotsa'ah.

Adding to the surprise, when the Mishnah lists the 39 melakhot in the seventh perek, hotsa'ah is last on the list. And the first Tosafot on the first amud calls it a melakhah geru'ah, an "inferior" melakhah.

So why the focus on hotsa'ah? Why does it get so many chapters? Why is it tacked on to the beginning of the tractate?