I think you’ll find that we’ve got a good explanation for at least some of it right here:

Was I the only one who gazed sentimentally at Yvette Cooper and Ed
Balls, marvelling at their becoming the first couple to be
simultaneously in the cabinet, before being rudely interrupted by this
thought: who’s going to look after their three children? Gordon Brown
has effectively orphaned them! While I say this somewhat tongue in
cheek – I’m sure there is an extended family only too happy to fill in
the gaps – I also know that I, and many women like me, would never have
made that decision.