Why isn't slavery right because it is wrong?

No. That is what is called a tautology, when someone says that something is because it is. Some overarching reasons why slavery is wrong are based on the concept of Natural Rights, which is rather widely accepted. This list is not exhaustive.

1) Slavery Degrades/Devalues a Person: Because a slave is the same as any other piece of property, it is valued at an exacting amount subject to negotiation. This means that a person's personality, age, strength, and other discernible aspects of a person acquire monetary value. This runs counter to the Natural idea that all humans exist beyond the ability to be quantified monetarily and are more valuable than their aspectual traits.

2) Slavery Inhibits Motion: This is rather obvious, but one Natural Right is the ability of person to choose wherever he wants to be and to be capable of getting there. (This does not mean that his travel is compensated, but just that it is permissible.) A slave, since he is property, cannot control where he is allowed to go or not.

3) Slavery Breaks Up Families: Since slaves were not sold as nuclear families, men were taken from their wives, children from their parents, and entire families were ripped apart. This is a violation of the Natural Right for a person to live in peace with his family intact.

4) Slaves Have no Right to Self-Protection/Defense: Since the slave is the property of his owner, he cannot try to defend himself from savage treatment by his master. There is also no regulatory agency or program to which a slave could apply to intercede between himself and his master. This made slaves completely at the will of their masters' temperament.

5) Slaves Have No Just Compensation: Since the slave performs a lot of labor and only receives some food, he is not being compensated at anywhere near his productivity. This is a less serious infringement than the others listed above and still quite common in the Third World.

6) So-Called First Amendment Rights: Although not present in all forms of slavery, most forms of slavery deny the slave the right to practice his own religion and cultural behaviors, speak freely to his master or to anyone he so chooses, and are certainly prohibited from writing and assembly.