Why is this sentence using past participle of "had earned"?
This is an example of the past perfect tense. This verb tense is used to talk about actions that were completed before some point in the past. This is used particularly useful when retelling events that have occurred in the past, as it allows you to describe the sequence of events more clearly. In comparison. Ex. If I had woken up earlier today, I would not have been late for school. Ex. The old hotel was abandoned now. It had once been famous for serving celebrities and politicians, but now only spiders and mice could be seen wandering across its halls.