You can pretty trivially expand the question as to why woman are choosing their careers over children.
Anecdotally, my other half would love to have children but I want to delay it right now because of finances. We live in the south-east of England so this is how things look:-
We have a nice 2 bedroom terrace that costs £500,000. I earn a good living but we're hardly living in luxury. The bank of mum and dad aren't rich, thus I struggle to pay that down. This could perhaps be attributed to woman joining the workforce, a lack of building, and a political lanscape that setup property as invesments.
She wants children to go to private school that may cost £9000-£30000 per year, with an oppertunity cost of upto £2,000,000 per child. This can probably be attributed to the nigh on impossibility to get your kids into a good performing state school and grammer schools being given the shaft after our parents had their turn.
She wants them to have a dad, but Men are afraid of commitment due to the litigious nature of break-ups which screw them over. I'm not saying Dads shouldn't pay for their kids, just they shouldn't be paying £4,000,000 for an ex-wife to keep the advantages of marriage whilst the man no longer gets anything in return
There's probably many more problems because the birthrate is fairly low right around Europe but these are some pretty big negatives for middle class men to commit to having children, at least not until they have their lot in life, which means there's less time to have more than 1-2.