1. The amount of fighter based competition has increased (WT,Il2, WOT, ect )
2. The graphics (as of now) don't attract new players.
3. Financial reasons (costs) as snailman mentioned.
4. Banning of long term players.
5. Eny, side switch times, size of the map in game play.
6. Many children can't handle the complexity of the skill this game takes or the patience.
7. 2 weeks is not long enough for the majority of people trying to figure out how to play this game with the <3 hours they have to play after work/school.
8. Marketing/advertising.
All of these things add up.
Edit: AH is like the Katatonia of the music world. Amazing in almost every aspect of their musicianship and range, yet are not well known and have a small fan base due to their pride of not signing a record label.