Here's why:
* Direct objects receive the action of the verb directly. They are the "who" or "what" of the verb. These objects are marked with the accusative "-n".
* Indirect objects receive the benefit or consequence of the action. They are the "to whom" or "for whom" of the verb. They are not the direct target of the verb's action.
Example:
* Direct object: Mi donis la libron al mia amiko. (I gave the book to my friend.) *Libro* is the direct object, receiving the action of *donis* (gave).
* Indirect object: Mi donis la libron al mia amiko. (I gave the book to my friend.) *Mia amiko* is the indirect object, receiving the benefit of the action.
Therefore, indirect objects in Esperanto remain in the nominative case, without the "-n" suffix.