Troll's clothes?

[Hot, crispy nice hobbit]

I think that before we can answer the question about why the Trolls' clothes turned invisible, we should probably wonder why the Trolls' themselves turn invisible.

Dear Darlin', you have gotten quite messed up between 'invisible clothes' with 'invisible Trolls'... I admit that the fault is mine: bringing up invisible clothes on a topic for Trolls' clothes... Goodness!

I take it that you mean that why trolls turn to rock upon being blasted with sunlight? Well... the eminent Professor Morgoth fabricated them, and I guess he thought it nice to make sure they do not stray far from his labs...