Because now the timeline has to bend over backwards to explain how she fits into certain events. For instance
How would she have handled Ray Palmer taking over her company?
What would stop Malcolm from trying to win her back?
Would she have been kidnapped by Chase and brought to Lian Yu?
And most importantly, what’s to stop Ra’s, Damien, Chase, Cayden James, Diaz, Emiko or Dante, or the Thanatos Guild from putting her in the exact same situation Slade had her in before she died the first time?