I must declare that I am not a specialist or have ever investigated in this field.What I am writing is just persional opinion,and is totally subjective.
For one thing,the prevalence of internet.The internet connects most of the strangers but estranges most of our kith and kin.While husbands fall in love with computer games and wives innovle in online movies,the link between family members becomes weak.
For another,the change of the concept of family.Double-parent with no-child,which is widely accepted today,is obviously a preposterous idea in the past.No child means less obligation and less hesitance during the progress of divorce.
I hope my answer would be helpful.