My own husband ran with another girl after I did all of these things. My opinion is that the answer is veey simple: selfishness. They see what they want and they take it. Another answer could be that the other woman is a "better enabler" which means like zoso says... She has much weaker boundaries.
I am sorry this is happening to you. I could have written your post. It is almost scary how similar your situation is to mine. Hugs.