Short answer: Because it's AWESOME. Long answer: Arakawa is brilliant. Everything in the entire story is thoroughly researched and meaningful to the universe, characters, and plot. She knows the perfect balance of action, drama, and comedy. The characters are complex and well-developed. The setting is even MORE complex and well-developed. The plot is fascinating, intriguing, consistent, full of twists and turns. Her artwork is great; unique and full of life and action. The animators recreate her style well. The (Japanese) voice actors are all incredibly talented and perfectly cast. The sense of dramatic timing in both the writing of the manga and the anime are spot-on. The alchemy system is beautifully designed, fully researched, and thorough. The story has no inconsistencies.

I think that about covers it...