The exchanges earn the interest if the HBD is held on an exchange. In theory they can pay it to the users (or only do so if users put it into a staking pool or such) as they do for other coins, but no exchange currently pays interest to the users for HBD. The users would only earn interest if they withdraw from the exchange, which doesn't seem to happen a whole lot if one looks at the balances on exchanges (high and stable if not growing).

So this is another reason why hardly anyone if anyone is buying and pushing the price up due to interest.