Because one anime episode can be as long as 4 or 5 manga chapters it catches up with the manga pretty quickly. When that happens you can do one of two things:
1. Add fillers into the anime until enough time has passed to continue using the mangas story. Like Naruto, Bleach, One Piece etc.
2. Deviate from the manga and go along a different story. This usually ends with the anime "ending" whereas the manga is going on strong.
I don't mind it either way, but I find that fillers are the best of going about it. Then again if option 2 is taken you can always, at a later stage, pull an FMA. The original FMA anime finished years ago, but they decided to redo everything by following the manga completely and call it FMA: Brotherhood. Which was fantastically done.





