Are you referring to the claim that Afghanistan is a strategic region that empires have fought over for centuries? Documentaries often say things like that. Just ignore it. It's a dumb cliche that needs to die. Wars happen everywhere.

It is not really the case that Afghanistan gets invaded to an unusually high degree. The two invasions you mentioned happened because of particular historical circumstances, not because of some eternal quality that Afghanistan possesses.

The motivations for the Soviet invasion are a little murky, but it essentially happened within the context of the Cold War with the Soviets trying to keep Afghanistan in their orbit. (See Hammond, Red Flag over Afghanistan, chs. 9-11; Tanner, Afghanistan: A Military History, ch. 9). This was in the context of the global competition between the two superpowers. There were several wars in the 1970s and 1980s that either happened or were exacerbated by the Cold War (e.g. Ethiopia, Angola, Lebanon). In this case, "what's so special" about Afghanistan was simply that it shared a border with the U.S.S.R.

The U.S. motivation for going into Afghanistan is obvious -- to eliminate al Qaeda. To do so meant toppling the Taliban regime. For a source, see the text of the joint resolution passed by Congress on September 18, 2001 (S.J.Res. 23). Afghanistan was the target because its government at the time happened to be harboring Al Qaeda. If the Sudanese government had been harboring al Qaeda at the time, then the U.S. would have invaded Sudan instead.

The narrative that Afghanistan naturally invites invasion and therefore is naturally prone to warfare is pernicious, because it can potentially feed into some destructive cultural perceptions and policy decisions. If a country is war-torn by nature (which is often said of Middle Eastern and African countries), then that can absolve people in the developed world of responsibility for that warfare (through how they vote and what they buy). It can also feed an unjustified sense of cultural superiority among people in industrialized countries.

Edit: Just to clarify, when I say the narrative is pernicious, I'm putting blame on the TV writers who perpetuate it. I'm not saying the OP is to blame for anything.