When Luna was reformed in Season 1, she was born with a low energy level.
Over the course of the series, her powers came back, and in Luna Eclipsed, we get to see her real form.
All this information was taken from Lauren Faust.
It might have changed because she was younger back then. I think that alicorns have their mane/tail changed when they grow elder, such as Celestia. It didn't happen to Cadence, though, so it's a mystery. I hope I helped. : )
I agree with Karina's theory. Cadence and Twilight are significantly younger alicorns than Celestia and Luna. I also think, when Luna was restored to her old self, her magical aura was drained, that is why she had normal mane, not like the awesome billowing hair. On a more realistic level, the character designers redesigned her appearance to match Celestia's design a little more.
Luna & Celestia have Mary Sue Hair; a common trait of poorly developed OCs in fanart used to make their characters special & otherworldly. I guess it's okay, seeing as they're goddesses & all.
