This question is something which is worrying- given the prevalence of this practice. It is plain cold blooded murder, but we don't look at it that way, we all know. One of the probable reasons I had mentioned earlier was that we are so sick of watching the plight of the girls around us (and the plight of their parents too) that probably this one time suffering is easier and more acceptable than a lifetime of suffering. But for some people it is a more mechanical decision. How?
I have an explanation, probably it is a lesser crime for our society, the way killing a mosquito is treated differently from killing an elephant, we probably think that since human foetus is smaller and has lesser level of consciousness we may get rid of it without any baggage of guilt. There could be another reason of this hardness of mind.The technology nowadays is very sophisticated. You detect the sex of the baby, you decide you don't want a girl, you tell the so called doctor, s/he performs the procedure which has hundred percent efficacy, such successful application of latest technology makes it a mechanized process, where the people involved probably easily sidestep the fact that the decisions, the procedures and the execution involves a human life... The parents feel that they are after allowing their baby girl a much easier end, better than carrying it to full term pregnancy and then strangulating it (such cases were in news, not so long back)! By selecting whether or not to 'keep' the baby, mother too saves herself from physical stress and the attachment which all mammals develop as a result of carrying their babies inside them for a gestation period.
Do we have a way out, things have come to a really hopeless point. I guess merely knowing the exact reasons, or so i think, solves no purpose... we need to work out a solution to this problem. Yes declining sex ratio is a real practical (apart from moral and social) problem. I'll discuss it in some other post.....