What was the official position of the Vatican during WWII and why is it rarely discussed?

I don't know that the Vatican in WWII is rarely discussed, as there have been a number of books on the subject. Cornwell's 1999 book Hitler's Pope ignited what is now called "the Pius wars," in which a number of authors hotly debated the pontiff's legacy. In this slightly out of date /r/badhistory post I give a somewhat acerbic review of the main contributors and their dozen books on Pius XII. I will return to that subject in the closing of my post, but I will answer your main question first.

The official position of the Vatican during WWII was neutrality. This was a treaty requirement of the 1929 Lateran Accords. Here is my older series of posts on the subject. Pius XII chose to honor his treaty obligations of neutrality in foreign affairs and focused on diplomatic appeals to attempt to mitigate the suffering. More pointed public appeals were considered, but the Vatican felt that such statements would exacerbate the suffering rather than bring it to a halt (reference the Dutch bishops' protest of Nazi practices and the resulting crackdown on Jews in that nation). From the very beginning of his pontificate in March of 1939, Pius XII attempted to urge individuals to act according to their conscience via papal statements. In his first encyclical, Summi Pontificatus, in October of that year (shortly after the German invasion of Poland) he made reference to his earlier appeals for peace (his Easter message in particular) and began what was to become a series of appeals for noncombatants to be spared from the horrors of war.

Venerable Brethren, the hour when this Our first Encyclical reaches you is in many respects a real "Hour of Darkness" (cf. Saint Luke xxii. 53), in which the spirit of violence and of discord brings indescribable suffering on mankind. Do We need to give assurance that Our paternal heart is close to all Our children in compassionate love, and especially to the afflicted, the oppressed, the persecuted? The nations swept into the tragic whirlpool of war are perhaps as yet only at the "beginnings of sorrows" (Saint Matthew xxiv. 8), but even now there reigns in thousands of families death and desolation, lamentation and misery. The blood of countless human beings, even noncombatants, raises a piteous dirge over a nation such as Our dear Poland, which, for its fidelity to the Church, for its services in the defense of Christian civilization, written in indelible characters in the annals of history, has a right to the generous and brotherly sympathy of the whole world, while it awaits, relying on the powerful intercession of Mary, Help of Christians, the hour of a resurrection in harmony with the principles of justice and true peace.

What has already happened and is still happening, was presented, as it were, in a vision before Our eyes when, while still some hope was left, We left nothing undone in the form suggested to us by Our Apostolic office and by the means at Our disposal, to prevent recourse to arms and to keep open the way to an understanding honorable to both parties. Convinced that the use of force on one side would be answered by recourse to arms on the other, We considered it a duty inseparable from Our Apostolic office and of Christian Charity to try every means to spare mankind and Christianity the horrors of a world conflagration, even at the risk of having Our intentions and Our aims misunderstood. Our advice, if heard with respect, was not however followed and while Our pastoral heart looks on with sorrow and foreboding, the Image of the Good Shepherd comes up before Our gaze, and it seems as though We ought to repeat to the world in His name: "If thou . . . hadst known . . . the things that are to thy peace; but now they are hidden from thy eyes" (Saint Luke xix. 42).

Since it is impossible to discuss WWII without discussing the Holocaust, it is also important to point out that opposition to violence based on race was not a novel concept within the Vatican. In 1937 Pius XI had released Mit Brennender Sorge, his document regarding the Church's stance on fascism. It was written by his Cardinal Secretary of State, Eugenio Pacelli--who would be elected as Pius XII a mere two years later. It contains this quote in paragraph 8:

Whoever exalts race, or the people, or the State, or a particular form of State, or the depositories of power, or any other fundamental value of the human community - however necessary and honorable be their function in worldly things - whoever raises these notions above their standard value and divinizes them to an idolatrous level, distorts and perverts an order of the world planned and created by God; he is far from the true faith in God and from the concept of life which that faith upholds

There are a host of books on Pius XII and his actions during the war. Most are utter garbage. The authors of these books used Pius XII as a bludgeon for current political causes--and that goes for both his detractors and his defenders. The one book that I can recommend without reservations is Robert Ventresca's biography of Pius XII, Soldier of Christ. Rather than being a polemic or a hagiography, Ventresca's book is a work of history. I do not agree with all of his conclusions, but they are at least based on the available evidence and are well-argued.

I hope the above--along with my earlier post--answers your question. As always, followup questions from OP and others are encouraged.