Because it sounds like there's enough drama with just the residents of Atilan and the Royal Family alone to get plenty of stories for a show like this.
Pure story considerations are fine for prose or comics, but on television, it's always a matter of what you can afford, and what unimaginative, cautious executives can be talked into taking a chance on. This is why so many fantasy and comics-based shows are police procedurals. This is why Syfy's Flash Gordon spent most of the first half of its season on Earth rather than Mongo. This is why the MCU began on television with a show about non-powered government agents, and why Netflix saved the most fantasy-based of its four Marvel shows for last. Network execs -- and the general audiences they're programming for -- need to be eased into exotic or unusual premises. Even a show that plans to go somewhere wild and way out needs to start in a more grounded place, or the execs will be spooked and afraid to risk spending money on it.
So yeah, it's possible The Inhumans could do a season set mostly or entirely in Attilan -- but I don't think it would be their first season.