It's not necessarily that that's actually the case, RitaM, but that people (including the people in positions of power in these organisations) assume that it'll be the case.
In my family, it's more likely to be the other way around (I earn more than dh, have a job where I'm likely to work long hours, blah blah, and dh supports me in that by being available to pick the kids up, cooking for all of us, etc etc.), BUT I am still seen at work as a 'working mother' in a way that my male colleagues are not. So if I'm at work at 7.00 p.m., I'll get comments about how my kids must've forgotten what I look like, whereas my male peers, even if they have kids of a similar age, do not get these comments. It's therefore assumed that men will be more 'available' to take these more senior posts on than women - often without asking the women in question.
Linked to this is the 'I dont' know how she does it' type comment, whereby women (especially mothers) who are ambitious and successful are seen as somehow exceptional (often also by other women). This means that more junior women coming in are not encouraged to pursue such ambitions, because they are not seen as being realisable by 'a normal [female] person'.
I am now resolved to challenge these assumptions whenever the opportunity arises, but - as I said before - it'll take a lot of women doing this before the system changes.