When only people who want independence vote on the referendum, obviously you are going to get such results. If the people opposing independence don't consider the referendum to be legitimate (since you know, it is illegal and against the constitution, and not recognized by the central government), then they are not going to vote on it. And hence the question, why doesn't the Spanish government allow it? Because it is an issue that affects all Spanish citizens, therefore, we should all have a vote on the matter.