The first is pretty easy. It's an extremely thoughtful show that cares about a lot of parts of humanity and culture that not many others do and it's ambitious while getting it right more often than not. It understands fascism and eugenics and why and how they rot us in a very fundamental way. It's just a great show even with its unevenness and age. I find those flaws charming.
Anyways the second reason grows from that last part. It's flawed, all its characters are flawed, it's deeply imperfect. It's a very human show about the way we see and treat each other and what makes people turn against each other. It's proof you can make something great without it needing to be perfect. It's a reminder that making something with other people, even if you make mistakes or don't hit the ideal vision, is beautiful and worth it. And I love it for that.