Obviously there are lots of reasons but here are a few:
1. Women usually outlive men 5-7 years so widowhood is a big factor. There are an estimated 258 million in the world according to the United Nations article.
2. Divorce is a major factor. How many times do you hear that someone has been married 25 or 35 years and suddenly they start a new life? It happens quite often and starting over after 55 can be a disaster or a very freeing experience. Divorce is definitely one of the big factors in the number of senior women.