I only watched FMA because all my friends were into manga and I wanted to watch one so I'd know what they were talking about sometimes.

The reason that the original series is different from the manga is that the manga wasn't finished when they were writing the series, so they just went off in a completely different direction. And it turned out that I liked some of the characterizations and plots much better than in the manga.

Sometimes the badly written fandoms are easier and more fun to write fanfic in.