How could the other woman still take him back after all he has done? I don't get it?

I am still at a loss for understanding regarding this: Its been since Aug30 that I found out about my exN and other women and that the whole relationship was one big farce. I contacted the other woman whose phone number I happened to find on his old cell phone bill. I reached out to her in hopes to prevent her from going thru what I went thru. Despite her know that I was pregnant by him, had an abortion, was living with him, him claiming he wanted to marry me, blah, blah, blah...I could hear the desperation in her voice as if saying "please don't take him away from me...". I was not a competition to her, I was trying to inform her...yet she viewed me as competition and chose to stay with him. He is now in prison for not paying $60 thousand dollars in child support...she is still with this guy doing all his legal work. I just don't understand this. One this I had observed was that he would make seem as if "us against the world", yet the problems consistently stemmed from the internal; with him being the common denominator...