What is it with some people and they stay in a ...

Hi Angel,
You raised a very good issue.  You are so right.  People stay in such bad / wrong / miserable relations for many reasons (each case for it's own merits): love (that "makes them "blind", for the sake of the children, being afraid to stay alone, financial insecure, hope things will get better, hope it is only a temporary situation, don't have the courage, afraid to get the title "divorced"..... and sure there are many more.
The best advise I can give to those people is: You live only once and there is no reason to live in a misery or in a lie....  be brave and strong and do the right thing (deep inside you know what you have to do). 
Best regards,   Sa Go Pa (= I love you, in Greek).

It's called co-dependency.  There are hundreds of books written about the subject.