Well, there's several things here: It's true that large parts of Mein Kampf concerns South Tyrol, though the entire issue features much more prominently in the so-called "Second Book", which – unsurprising given the name – is Hitler's Second Book. It was dictated in 1928 to Max Amman, who was the head honcho of Nazi publishing and buisness manager of the Nazi party. The Zweites Buch, which was never published during Hitler's life time, was primarily concerned with Hitler's thoughts on foreign policy and a huge chunk of that concerned itself primarily with the South Tyrol question and how it would affect the völkisch movements realtionship to Fascist Italy, which at this point – more so than when Mein Kampf was dictated and published – had become an important point of identification for the fascist movements in Germany.

But let's start with Southern Tyrol: Many people will most likely be very familiar with the Versailles Treaty, which reduced Germany in territory and included several provisions and prohibitions concerning Germany, its army, and so on. Versailles was pretty much the center piece of the Paris peace treaty conference. But Austria too signed a peace treaty in one of the Paris suburbs. The treaty of Saint-Germain also declared that Austria was, together with Germany, responsible for the war and would have to pay reperations (which they ultimately didn't pay), was not allowed to unify with Germany and also had to make territorial concessions: Primarily, it had to cede a lot of territory in Bohemia and Moravia to newly founded Czechoslovakia, had to allow plebiszite in various territories to see where they wanted to belong (parts of South Carinthia went to Yugoslavia, parts of Hungary went to Austria), and had to cede a variety of territory to Italy.

That terriroty included Istria (later to be ceded by Italy to Yugoslavia and now part of Croatia), Trentino, which the Austrians also called Welschtirol and, crucially, Southern Tyrol. Crucially because while the argument can be made that the Trentino and to an extent Istria were majority-wise in terms of population Italian-speaking, or at least, not German-speaking, South Tyrol was unequivocally a territory in which the majority of people spoke German and identified as German(-Austrians). This is also a major factor why the newly created Austrian Republic was, if not quick, then at least willing to accept that a return of the Bohemian or Moravian territories was very unlikely and the needed to be ceded, took a long time to accept South Tyrol as part of Italy. South Tyrolean irredentism continued to exist in both Austria and South Tyrol all throughout the 1920s and 30s and even beyond the Second World War. It even came to several waves of irredentist terrorism by South Tyroleans against the Italian state by a variety of groups, including Neo-Nazis, up until the 1980s. However, the end of the South Tyrol issue – though it sometimes resurfaces even to this day – came in the early 1970s with the so-called Zweites Autonomiestatut, which greatly expanded the autonomy of South Tyrol within the federal structure of the Italian state and can be viewed as the most favorable settlement of the issue for the people of South Tyrol, who enjoy political and financial advantages from both Italy and Austria (and the EU).

Back to after WWI: Having to cede territory and the cause of the "lost" territories was a huge cause for the extreme right after WWI. The völkisch movement, born out of a Pan-German movement of the 19th century and fusing pan-German nationalism with anti-Semitism, had even before WWI been obsessed politically with unifying "German" territories, most importantly Austria-Hungary and the German Reich. The ceded territories in the East were important here but South Tyrol as well for that was not only lost but also lost due to what they perceived as treachery, when Italy left the middle powers and joined the Entente in the middle of the war. Furthermore since the territory had become part of Italy, the fascist Italian government had embarked on a campaign of forced Italianization in South Tyrol. They surpressed German language institutions, German language and so forth and tried to settle Italians in the territory to strengthen the Italian claim on Soputhern Tyrol.

This is among the main reasons why it became such a cause celbré for especially the völkisch right movement in Germany and Austria but also beyond that movement politically. The conservative right in German speaking countries and even the left was pretty outraged by the programm of Italinization and more than almost anything else, it stood for the hypocritical position of the Western powers, and especially the US and Wilson, when they talked about the principle of self-determination. How could they talk about self-determination when Italy was allowed to italiniaze South Tyrol?

In this sense, when Hitler first dictated Mein Kampf, it was an issue that was obviously something he felt strongly about as being born in Austria and working politically in the völkisch movment in Germans and one that he really wouldn't be able afford to not address. To draw a modern parallel: It was the one issue in 1924/25 that no political could afford not having an opinion about, similar to how modern day US politicans always have autobiographies ghostwritten in the run up to the campaign that also have to address a couple of things like foreign policy vis-á-vis China.

This is important background to why Hitler felt so compelled to expand upon the issue in the Second Book in 1928. By then, Italinaization was in full swing in South Tyrol and this caused great controversy in teh völkisch right, especially since parts of the fascist right in Germany advocated strongly at that time to emulate the model of Italian fascism and closely cooperate with the Italian fascists, who were seen as the first model of success for fascists movements. Hitler had also advocated for this position and wanted close cooperation between German Nazis and Italian fascists. This opened him up to criticism within the völkisch political spectrum in Germany and various people had attacked him and the Nazi movement for this position and accused him of being willing to sacrifice the German-speakers of South Tyrol to Mussolini. That's why he felt the need to justify himself and his position at greater length and within a larger framework of his ideas of foreign policy.

It was also among the reasons why the Second Book was never publishied during Hitler's life time. Not only did his publisher recommend against brinign out a second book that would compete with Mein Kampf but also new issues such as the Yong Plan replaced South Tyrol and the Italianization as the most important political cause of the political right in Germany at the time.

Ultimately, both the Austrian fascist, who sought close ties to Italy, as well as the Nazis once in power settled the South Tyrol issue to a certain degree in a way that worked for them. While the Austrofacists chose to let sleeping dogs lie, Hitler and the Nazi regime tried to settle the issue by offering to take any and all willing Southern Tyrolians who spoke German from Italy and planned to resettle them in the Soviet Union. For obvious reasons those who opted for the German Reich, the so-called Optanten, never made to the Crimea but got pretty much stuck in Innsbruck. These people would later become a huge force in Southern Tyrol irredentism and terror campaigns but there weren't as numerous as the Nazis had initially hoped for and while that settled the issue somewhat politically between the two fascist powers, the issue came up again once the war was over.