I think it's a combination of several factors.
First off, we are all supposed to be sisters, and a woman who goes after a man who is already taken is breaking an unspoken rule. Most of us would not do it ourselves, so we get angry at a woman who will.
I also think that blaming the man is just too hard on our psyches. If we blame him, we may be admitting all sorts of things, like the fact that he has little consideration for us, our judgement may be lacking in having picked him in the first place, there must be something that she has to offer which we don't, and that our trust in him and our time with him may have already been compromised before, which makes us a fool. I think it's just easier on us to blame a tempting, evil seductress than it is to take that much assorted blame on ourselves.
The last factor, I believe, is just the fact that our self-esteem takes such a beating when we find out we have been cheated on. We are so beaten down, we are afraid to get really angry with him, because he might leave. We're so awful, who else could love us? The thing we cannot see is we are usually better off without his sad butt in the first place.
The irony, of course, is that when our men cheat, even blaming the other woman doesn't spare us all our grief. We still blame ourselves, when what we really need to do is blame the responsible party--our man. He's the dog that follows his nose wherever it leads, he's the one who hurts us, he's the one who has the power to say no to the other woman but doesn't, he's the one who tramples on us like we're dirt and thinks it's okay.
參考: Was with a cheater once, and put a lot of thought into why I always blamed the other woman, and why I always took him back
It might be because we feel that this other woman has something that we don't... It's a horrible blow to our self esteem to feel that at that moment... he had to have her... and didn't love us enough to feel that it wasn't worth it. That she might have had some power over him. That SHE made him cheat.
The real truth though, is that the man made a decision when he chose to cheat. Life is all about decisions. And whether he had to have her at that moment or not... was a decision that HE made.
參考: Second paragraph - My husband... who has & won't cheat on me.
coz we always think they r sl ut, n find it easier to accept this cruel reality tht our man find someone else, we pretend tht its the womens fault but mostly i think both of them r wrong