I know soooooooo many women who only take part because it's non-threatening and actively encouraging for women. It allows the unfit, the unwell, and those who have literally just off the couch to exercise in public without being subject to criticism. it isn't competitive at all - it's a fundraiser - that's not to say that people don't compete, but anyone who has actually tried to take part knows that sheer volume means you can barely manage a jog for most of the way.
I know no men at all that would be seen taking part, which is probably why the male equivalent flunked big time. That isn't to say that there isn't a need for men who are equally unfit, fat, or unsporty to be allowed to exercise in public without fear of being pointed at and laughed at, but that this is not the event to force the issue.
If RFL opened to men, the numbers of women taking part would plummet. It would lose the 'safe' aura, there would be a slack handful of blokes to replace the women, and the events would be cancelled, leaving a huge hole in the cancer research coffers.
In an ideal world, women wouldn't feel they needed a safe space, of course. But it's fairly stupid to deny that's the way many women do feel...
Fwiw, I've done a few, in solidarity with female friends weighing in at around 24 stone. There is no way at all they would have taken part in any other event. No way. I wear my 'other mother's' name on my shirt when I run RFL. She died ten years ago of bowel cancer.
If I run for me, I choose other events. (I ran the London Marathon and GNR for cancer research). There are plenty of them. So many that you can pretty much pick and choose something that fits your needs - and still raise money for CR.
There are places for female-only participation, and this one 'run/walk' is one of them. And I speak as someone who would generally fight to the death for non-gendered opportunity.