The "why" doesn't really matter much. All these shows where influenced by, but more importantly, influenced, the zeitgeist. Even though D&D wasn't limited in what it could do by TV budgets it still had to speak the language of the culture which had been defined by those TV shows.
So you can, for example, trace the Cold War Nuclear fears influence in Gamma World and Dark Sun 70s/80s, the Star Wars influence in Dragonlance and Baldur's Gate 80s/90s, LotR movie influence in the early 2000s, and so on.