Feminism is largely to blame. As per biological evolution, one of the traits women look for in men is the ability to provide.
Now that women are entering the workforce in record numbers, a common theme amongst working women is wanting to find a man that earns more than she does. A man that earns the same or less than the woman is often seen as not having the traits of a good provider.
The few men that actually earn good incomes and aren't hideous, generally have plenty of options, and if you have options. Why would you settle for a woman that is combative and jaded by years of being in the workforce?
Until more women decide to be wives/mothers, instead of strong independent hos, expect the birth rate to continue declining.
Obviously you have to factor in western societies downplaying of the role of men/masculinity, the complete bias in the family court system, and family/child support laws in general, because generally speaking. After blowing a load inside of a vagina, men have almost zero say in the birthing/raising of children.
Why have kids if all you are is a sperm donor/human ATM machine?