Why my akita's tail is dropped?

Because he doesn't feel like carrying it over his back. Most dogs with curled tails do let their tails drop fairly frequently--whether because they're sad, submissive, tired, frightened, or just in a neutral mood and don't want to curl their tails.

Akitas (and other breeds with curled tails) are more likely to curl their tails over their backs when alert, particularly confident, domineering, or excited.