The answer, in my mind, is twofold.
1) The sheer logistics of looking after a sick wife is life altering; not to mention, terrifying. My mother was terminally ill for many years; and, my father stuck with her every single minute. However, his life was changed forever. Mom was fairly immobile for a number of years; so, dad had to do 99% of what needed being done in the apartment. He also had to go with her to medical appointments; give medications; help her move from place to place; and, other things too numerous to mention. For some men, it's too much to handle or some just don't care to handle it and leave.
2) The husband can't stand to watch his wife in pain knowing the ultimate end is death. Some men do not have the stomach or the heart for this journey; so, they opt for divorce.
Kudos to people like you and my dad. I know my mother's life was made much easier by the loving care my father gave her; and, I'm sure your wife felt the same way. You are both good men - a rare commodity nowadays.