So as I was trying to go to sleep last night, I started thinking about a picture from The Mata Nui Saga.
And I remember thinking Why is Tahu in his Stars form and not his Mata form? And of course the first answer my brain comes up with is Marketing. But then I started to wonder why the other Toa didn't look quite the same. So then I started wondering if there was a canon reason for this. So I let my mind scour it's vast memories of BIONICLE facts and trivia, and I remembered something the Kanohi Ignika said to Tahu when it de-evolved him back to a Toa Mata. "In order to do what you must, you must be what you were" (That was in BIONICLE: Glatorian #6 All That Glitters if you want to look it up). I also remember that in BIONICLE Legends: Island of Doom, Lewa didn't want to get into the Toa Canister, and the Turaga reassured him (or at least tried to) by saying that due to malfunction of the canisters, they floated around Mata Nui for a thousand years, causing there organic parts to decay. So that made me wonder if some of the mechanical parts started to decay also, or at least change shape. My strongest argument for this would probably be the bicep armor that is on the Tahu Stars, but not on Tahu Mata. One thousand years of rust and degradation could have caused them to be more of a hindrance than a protection.
So when the Ignika de-evolved Tahu, it went with the form he should have had on Mata Nui, but didn't have due to thousands of years of stasis, causing him to degrade.
So that's my theory I thought up last night when my mind wouldn't shut up when I was trying to sleep. Any thoughts?