Why do you think this is:?
Athiest couples have lower divorce rates than religous couples
50% of interfaith marriages fail
My take is that atheists aren’t in a hurry to get hitched. They aren’t waiting for their wedding night, and they generally have been living together for some time when they wed.
If you haven’t lived with someone, how can you want to spend the rest of your life with them? That’s ridiculous. You barely even know them at that point.