Are we ever given a reason for why Phasma is so determined that no one know her past? It seems no one in the First Order would care or hold it against her if Cardinal's initial reactions are anything to go by. The risk of murdering/backstabbing people over this doesn't look like it'd be worth the effort.
- Because (although it took a while for Cardinal to actually cotton on) it could prove dangerous to her eventually. If someone knew all the facts that Cardinal knew they could figure out that Phasma's "loyalty" to the First Order is no more real than any of her previous affiliations. Huxes Sr. and Jr. never realised that she saw them as just another stepping stone and if she thought she could find a more powerful sponsor (or as in TFA her own safety was in danger) she'd sell them out just like she did anyone else. Someone like Snoke might figure that level of ambition is a bit too dangerous and arrange a "training accident" for Phasma herself. It's unlikely, to be sure, but there's nothing like a life of treachery and back-stabbing to make a lady paranoid...
