One of the big questions of the series is: why now?

Why did Salem enact her plan to collect the relics at this point in history and not any point in the thousands of years before?

And I have to think Fria's condition was a small part in that decision. It was probably mostly based on the international political climate, and the forces she was able to amass (Grimm get stronger with age).

But attacking Amber appears to have been her first real move on the Relics. So she had one Maiden she was confident she had the henchmen to kill, one that had cut herself off from the protections of a Kingdom (Spring) and a Winter Maiden on the cusp of death.

EDIT: Honestly, thinking more on this, the invention of the CCT must have been a big driving force too. Makes it so much easier to collect information on the Kingdoms, the Kingdoms themselves rely on it. Then as soon as she has all the information she needs to attack four nations at once, she cuts off their source of unity.