Cycle of abuse.
Pink was delegated to basically being a Spinel for the other Diamonds until they wanted to discard her but couldn’t. They made Volleyball, Spinel, Pearl, then gave her Earth, then tried to “put down the Rebellion” all just as part of increasing attempts to have her fuck off and work where they wouldn’t have to deal with her.
When Pink was given Earth she thought she would be like Blue from that point onwards. Meaning Spinel, what Pink was to Blue, deserved to be thrown in the Tower so to speak. But a handful of millennia to an epoch-old god aren’t a big deal like they are to a millennial.
Because Pink didn’t learn to consider the desires of other beings until Pearl confessed her fantasy, and didn’t learn that she could let herself be an equal instead of always thinking of herself as a garbage-tier person, Spinel had to languish.
Although considering some of the cut show content like the Gems who “starved” from lack of sunlight while on the run, or the horror of spending most of history trying not to think or feel like White did to herself, or being mindwiped seven times like Rhodonite, Spinel didn’t get the worst fate.