The shares of the trust are freely tradeable, but the the underlying assets in the trust are not. Shares are created by depositing bitcoins into the trust, but those shares cannot be redeemed for the bitcoins in the trust. It is basically a one-way peg.
There are many possible reasons for why there is a discount, including the possibility that there will always be a discount.
In contrast, shares in an ETF can be redeemed for the underlying assets (by a so-called "Authorized Participant"). If there is a discount, then APs will buy and redeem the shares, and then sell the assets for a quick profit. This action reduces the discount by raising the price of the shares and (in a small way) lowering the value of the assets.
It works the other way for ETFs, too. If there is a premium, then APs will buy and deposit assets for shares, and then sell the shares for a quick profit. This action reduces the premium by lowering the price of the shares and (in a small way) raising the value of the assets.