House of the Dragon works because it has so few magical elements in it. It has dragons and that's basically it. The rest is just a medieval type family drama saga. Hard to mess up if you keep it as simple as possible like they do here.

The problem with modern fantasy shows is that they almost never set up the rules upfront and at the same time they throw in as many magical elements as possible to make their series stand out without setting up the rules on how everything works. At most, they go the lazy route and do tropes as in, "There are elves in this world. You know else, right? Pointy ears. Fast. Sleek. Yep. That's it to them." That's not world building. That is a paper thin set up of lazy tropes trying to pass themselves off as rules. And you can't do that with fantasy. People need to know what to expect in this universe. What creatures are in it. What is the hierarchy of them. What can magic do here. What can't it do.

Basically, it's work. Lots of work. That's one of the reasons why if you look at fantasy novels, they are usually so thick, they can double as a booster seat and almost always have a glossary of terms in the back and sometimes even a map in the front.

But modern television writers don't want to do this work. So, they end up with bad series that looks like every other bad fantasy series. Like one giant mud puddle.

The reason why sci-fi series don't have this problem is because, well, the rules for the most part are already written out. It's called science. Hence, science fiction. People already know what can and can't be done because most people have a basic understanding of science. They understand the "speed of light" is fast. That people can't survive in the cold of space without the right equipment. That aliens aren't going to have the same biology as humans even if they look exactly like a human. That time is linear. That if you go back in time and mess with it, that will lead something changing in the future. Nothing needs to be spelled out up front because school has already told them what they need to know.

Again, you can't do that with fantasy because there are no universal rules across the genre. What may work in one series, may no apply in another series.