Here's why:
* Pronoun Case: "She" is the nominative case, used when the pronoun is the subject of a verb. "Her" is the objective case, used when the pronoun is the object of a verb or preposition.
* Comparison: In the phrase "earlier than...", you are comparing "she" to something else. The pronoun is functioning as the subject of the implied verb "was" in the comparison.
Example: "She arrived earlier than he did."