Why shouldn't fathers post here?
Why shouldn't men generally post here?
Why shouldn't women who are not mothers post here?
It's just a chat room.
I am a mother of 2 (well, I say that but for all you know I could be a bloke living in the basement of his mother's house) but I spend practically NO time on here talking about child related matters. I bet if you added it all up, I've spent more time talking about which is the best fruit and cheese combo than I have spend discussing mothering issues.
People come here because this place has a lot of intelligent, articulate and just plain funny people. Why wouldn't men, or non parents want that?
I think people have to get over the whole it's called mumsnet so it has to be for mums thing. It may have started out as a way for mums of young children to find out where's good to take the kids but that's not what it is any more.
As to why it doesn't change the name. Brand awareness.
Samsung started out as a grocer. Then moved into textiles. Then into all sorts and is now the Samsung we know today.
You think they built up their name and then changed it every time the company evolved? No. That's a stupid thing to do. You don't spend a lot of time building up your brand and then rename it.
People hate that.
I still say Marathon, Jif and Opal Fruits. 
You think I'm going to stop calling this place mumsnet if Justine goes hmmm, well, we have people who aren't mums here now, so let's call it EveryoneNet. By people for people.
There are all sorts of people here, some parents, some not, most (we think) women, some men. And that's ok. We're just chatting.