I would argue that the decline of the family is directly linked to feminism, or better stated, the liberation of women from the home and from conservative sexual values. The two forces behind that are birth control and women entering the work force. These two are magnified by online dating and social media. I am not saying at all that these are bad things, just that we have yet to see the true effects of feminism on…
Notable mention for flat/declining wages since the average cost of raising kids probably doesn't get cheaper as time goes on. Imaging paying your you and your spouse's student loans, then having two kids and paying their student loans or putting that debt on their shoulders. Much easier before tuition skyrocketed. Healthcare prices are a consideration too.
So economics is probably a weak explanation. Personal freedoms, especially for women, is probably the root cause of low birthrates.