Why birthmother's choose a single parent?

I posted this question on one of the birthmothers' boards also. But maybe some of you have already adopted as a single parent. My mom had asked me why a birthmother would choose a single parent when adoptive couples are available. I'm not sure why one would. I couldn't answer her and I wonder if anyone has been chosen by a birthmother and if they were given a reason why she was willing to look at a single parent adopting her child. Any of you have any experience with this? P.S. I'm looking at domestic adoption. I can't afford to adopt internationally and don't like the travel requirements--I have a toddler son that I wouldn't want to be away from.:confused: