Why are my breasts no longer sore?

Probably hormones. It depends on why they were sore in the first place. If they were sore or tender before or at the start of your period this is a response to the hormones in your body at that time. These cycle throughout the month, making your breasts tender or sore at some times but not others.
I do not know. Until one knows why they were sore in the first place, we may never know why they are mo longer sore. But I am glad they are not sore anymore. Best wishes.