It's because in modern society men are universally vilified, both in the media and within domestic settings. Within the confines of an intimate relationship a man can commit at least a dozen criminal offences just by having an argument with their partner. The head of the CPS has stated publicly that the threshold for prosecution in relation to incidents of domestic violence, coercive or controlling behaviour, is nothing more than "a convincing narrative".
A woman can literally ruin a mans life with a single phone call. Take his kids, his house, even orchestrate a situation where by the man cannot see his kids at all.
This imbalance of power is very real and not something that a lot of women are willing to acknowledge. A good friend of my husband is currently living back with his parents and hasn't seen his kids morw than twice in the past year, his ex wife has ignored all contact orders, and when he went to her house after five months of not seeing the children, she called the police!
Yes, I am fully aware of how real domestic violence affects women, but that is a different debate.
Also men tend to be more analytical and concentrate on the outcome, rather than the process. So in a lot of cases they consider the circumstances and will decide that the issue isn't important enough to argue over. Their tolerance seems to be higher and they don't focus on little details like we do.
I look at it like this. One of my husbands best friends is a police officer. The guy comes across as passive and weak willed, if he was in a pub and some drunk idiot gave him the whole "what you looking at" speech, he would just walk away. Not because he IS passive or weak, just that because of his job, his tolerance is so much higher, and his confidence so much greater, that he just doesn't need to prove himself.
For men I think it's a combination of both issues. The fear of losing everything affects the more passive men, and causes them to remain meek and quiet. While the confident men see no need to assert themselves over what they see as trivial issues, so they do or say whatever will placate their partners.