User blog comment:FairyRaven/Fairy Tail Chapter 467: Mother's Key, Review and Commentary/@comment-86.66.207.52-20160104123322/@comment-7418318-20160106195812

Actually we don't know that.

The Royals were very confident that they could use a substitute for not having all keys, but that in return the Celestial Spirit Mage would be "sacrificed".

That she didn't have the full Zodiac set might be the very reason why Layla died from opening the Eclipse Gate. After all Hisui, F-Lucy and F-Rogue could all open the Gate without dying, but only after the Gate was unlocked with the full set, so why would Layla die from it, if she also had the full set?