Its explain in the Manga so I heard, but in the Anime its just Like this:
Ichigo powers are just to high.
It's kinda like Kenpachi Zaraki and his sword. Except Kenpachi Zaraki Doesn't know the name of this sword. Doesn't stop the fact that his sword is released. He's power is just strong like that.
You should wait to finish the show or Read the Manga.
To round it up Ichigo's Zanpakuto always in Shikai State.
I meant that it wont change back to normal because the user has enough spiritual pressure to sustain it. Like what Kenpachi Zaraki can do.