A lot of women stay in a marriage that is a bit shit, because as some posters have said; they get swept up in the early throes of a relationship that's pretty decent (as the man has not shown his true colours yet,) and before they realise what a twat he is, they have often got financial ties with him (debts, loans, mortgage etc,) and also children.
Also, women, in most cases, earn less than their husband, and their life - and career - is often affected much more than their husband. A woman's earnings often drop even more after having kids (some even lose it if they become SAHMs,) making it much harder for her to leave.
A mother is often the default parent for the kids to stay with when the marriage breaks down too. I have known many men leave their wife for another woman, and they NEVER want to take the children. Funny also, how they only ever leave their wife when they have someone else to go to... 
Many jobs pay shit pay, (especially ones that women are usually expected to have,) and many people are lucky to clear a grand a month. With private rent often costing two thirds of that, and council tax costing £150 a month plus, there is very little left, and no WAY can a single mother survive on that, let alone be comfortable ...
That's why many women stay in shit, dull, boring marriages with men they fell out of love with many years ago, because it's better than living alone and being below the poverty line. I know/know of about half a dozen women who are just sitting there, waiting for their husband to die.
They don't even like him, haven't had sex for more than a decade, and have hardly anything in common. But they are financially comfortable, with a nice lifestyle and refuse to give that up. A couple of them don't even work. They just live off his money. They're not going to give that up, just because their DH is a miserable grumpy fucker who they haven't fancied in YEARS and who bores them to death.
If there is abuse that's different, but many women stay in dead, dull, boring marriages to men they don't even like, because they would never survive alone, and they like their lifestyle. It's much more common than you think!