Marriages lasted in previous generations because 1) pre-marital intercourse and divorce were taboo, 2) house work(cooking, cleaning, sewing, child rearing, etc.) was time consuming and labor intensive to the point of requiring full time attention and 3) females were limited to being a nurse, teacher, secretary, stuardist, model, actress, prostitute or waitress while bread-winning jobs were reserved for males. Male/female gender roles and dependancy on marriage was embedded in culture and economy. The bread-winner was dependent on the home maker and the home maker was dependent on the bread-winner. Although these marriages lasted, they were often full of domestic violence, romantic dissatisfaction, social turmoil, etc with no end in sight. It was not practical for a couple to separate. The husband and wife were "stuck" together.
Marriages more often don't last in today's generations because 1) pre-marital intercourse and divorce are socially acceptable and commonplace, 2) house work can be done as a side job(thanks to laundry machines, microwave ovens/fast food, inexpensive store bought clothing, day care centers) and 3) bread-winning jobs are available for both sexes. It is practical in modern times for a person of either gender to work full time at a decent job, do housework and raise a flock of offspring. Aside from couples who are tied together due to financial dependency on being part of a dual income household, marriages aren't as well-rooted as they were in the past.