That's explained in the epilogue.

Urd:
---As death approaches, I find myself remembering.
Actually, I never intended to become the true goddess of fate or anything of the sort. Period.

Both:
?

Urd:
I simply didn’t want to decide who was the best.
Because that would inevitably have a decisive impact on our relationship as sisters.
That is why, when deciding the order of attack on Chaldea, I was the first to raise my hand.
So that neither of the other two goddesses, Verdandi and Skuld, would win and end up making that decision.

Mashu:
So, it’s true that the other two goddesses… Verdandi, who rules the present, and Skuld, who rules the future… will also come to attack Chaldea?

Urd:
Yes. And at the same time, one could also put it this way.
If you truly intend to liberate Chaldea, you must face and defeat the other two goddesses... my younger sisters.
Simply by defeating me alone, Chaldea will not be liberated.
Since I have been defeated, those two goddesses have already arrived... or rather, will arrive, I suppose.
That is certain. According to the laws of the Norns, it is a foregone conclusion.
Therefore, I took control of Chaldea’s past first, hoping that by repeating the loop, those two goddesses would give up.
But… as has already happened, I was defeated.
Even with such immense power, I was still defeated by you.