Bungie didn't want to create a straight-up sci-fi shooter with Destiny, instead it wanted to create something that wouldn't be confused with its Halo franchise, according to writer Eric Osborne, who said the team kept this in mind during the game's development.
"Really, the artists were trying to push hard away from sci-fi because of the Halo legacy and history," Osborne told IGN. "They were just thinking, ‘What can we do that's radically different after 10 years?' So there's actually some concept art that you can find online of a very fantasy-driven world of knights, swords and sorcery in a white city on a hill.
"That was very much pure fantasy, but the more they continued to work and the more their ideas formed over time, the more they realized that the lure of sci-fi was just something they loved and they were denying themselves that creative space. So they thought, ‘What if we just take these two things and smash them together?'"
To sum up: they were afraid of being generic, so they made a game that is both scifi and fantasy, admitting that Star Wars influenced that idea. Which makes sense considering you can wear what is basically a Mandalorian helmet in Destiny.