1) Reliable birth control.
2) Changing social norms (100 years ago you were wierd if you didn't have many kids, now we put you on reality TV if you have many kids).
3) Wealth. When you split nothing 10 ways, you still have nothing. Meaning you gave up nothing. If you had $10000/year income just yourself you were dirt poor but if you split that 10 ways, you were still dirt poor. No real change to your lifestyle. Now that we are wealthy, we give up a whole lot to have a child. A $100k salary is quite nice for a single person but splitting it up 10 ways now means you go from a lifestyle of near luxury ($100k each/year) to a lifestyle of near poverty ($10k each/year).
4) Feminist revolution. Not too long ago the only way for a woman to survive was to latch on a man and sire him a son. Now women can do quite well without ever having a child.
5) Travel & entertainment. What else did you do for fun in the 1700s but create children? Now we have tons of forms of entertainment. Now we can explore the world and do great things, that is, until we have kids that pretty much leash us to a narrow boring 30 mile radius from home.
6) World trade, insurance, and social security. Not too long ago the way you survived a bad period of weather was to have a ton of kids toiling hard to force something to grow out of the ground and feed you. Now, we have insurance to cover the crop loss, use that money to buy food from places around the world with good weather, and count on the government keeping you alive through our 70s, 80s, and 90s.
I could go on and on.