Why is it that "no means no" used to be the popular anti-rape slogan and now the new slogan is "not yes means no"?

I'm still struggling to understand why women can't say no or otherwise indicate they aren't interested in sex.

The common answer seems to be that maybe a woman is too scared to say no or something along those lines. Shes too scared of saying no so shes willing to be raped instead because of what MIGHT happen if she shows unwillingness?

Seems like a huge pile of shit to me, sorry. I've also been with one woman for about 10 years. This isn't something that really affects me personally. I just find it amazing how quickly things have changed.