Because class warfare, the rich oppressing the poor, and the poor trying survive in a world run by the rich and powerful arent unique to any single time period. Stories like robin hood and King Arthur can take place in a futuristic setting as easily as a medieval setting. Or on any planet populated by sentient species. And the magic of king Arthur can exist alongside scifi tech or be replaced by scifi tech.
It's the same for a bunch of other timeless legends and myths. They all contain universal ideas, characters, plots and themes that can be transplanted to other worlds and times. Pretty sure Joseph Campbell wrote a famous book about this.
I understand the themes work in anytime, but that doesn't mean Robin Hood is the right story for that time setting. It's medieval folklore. Taking it out of that environment smacks of trying way too hard.