First, I'll address the first question, since you already partially answered it. It was called the "graveyard of empires" because Afghanistan remained nominally independent during the so-called "Great Game" between the Russian Empire and the British Empire in the 1830s when its neighbors like the Kokand Khanate in the north were being annexed by the Russians and its Indian neighbors were being done the same thing by the British. A more extensive explanation could be found in Seymour Becker's book "Russia's Protectorates in Central Asia: Bukhara and Khiva, 1865-1924", but in summary, Alexander II, Tsar of Russia, agreed to a neutral zone (which is to be modern-day Afghanistan) in between Russia's central asian protectorates and Britain's crowning jewel, the Indian subcontinent, in exchange for a guarantee of mutual non-interference in each of the empire's then-current spheres of influence. Thus, it was in Afghanistan that Britain's and Russia's imperial ambitions in Central Asia ended.

As for the second question, there is a lot of factors to be considered, but in the shortest way I could put it (FROM http://www.geocurrents.info/geopolitics/the-afghan-%E2%80%9Cgraveyard-of-empires%E2%80%9D-myth-and-the-wakhan-corridor);

1. The Hindu Kush Mountains
2. The Pashtun's warlike culture

P.S. Even as a history dilettante, I hesitate to use the term "graveyard of empires" when describing Afghanistan and the general region it occupies today and the other locales it used to occupy in the past, since Afghanistan in many iterations was conquered by quite a lot of peoples, the most significant one being the Sunni Muslims of the Umayyad Caliphate, but that is a tangent for another time.