I can imagine a few sadists who'd love to say it was because she was locked up for trying to attack Sally on her wedding day with Sonic or locked up when she went nuts when Sonic didn't marry her, but such immature jeering aside, I do have my own theories.
1) In the original timeline since we don't know when Eggman went down or Sonic got married, after Sonic and Sal got hitched, Amy with no other purpose left besides: threw herself into more and more dangerous missions and battles, not so much seeking death but because her combat talents was the only thing she had left, and eventually the law of averages gave her to the reaper.
2) In the King Shadow timeline, Shadow at one point early on tried to eliminate Sonic as a threat to his police-state utopia, but Amy, always the devoted, attacked Shadow in Sonic's defense, and paid the final price for her unacquainted love.






waaaaat