A lot of it is because women traditionally used to start giving birth at around 16-18, when you are much more fertile. With women now having careers, there is pressure to delay motherhood until the thirties, at which point fertility is already declining.Also some problems such as endometriosis get worse with every period, so whereas in the past you might have had only a few periods your whole life, due to numerous births and breastfeeding, these days you have most of your periods, and so the disease progresses. Endometriosis is a major cause of fertility problems.Also there will be some skewing in figures as with the solutions available women now identify and seek solutions for fertility, whereas in the past it might be a shameful secret, and with no medical solution you would just have to accept it.