It's easier to take care of. Old women don't have a time to make hairstyles and stuff like that. Also, I think sometimes the hair follicles become weaker as they age and they will suffer from too much hair loss, and usually the longer hair strands tend to fall more easily than the shorter ones. So cutting their hair will reduce this hair loss.
Probably cause they consider it to be a more mature look. The biggest reason has to be the thinning of hair and having a cut that can hide bald spots. To hide balding, you need to have a hair cut that you can easily give body to. Also when you get older, you have less patience to care for long hair.
Like sfauo said, probably so it is easier to take care of!
But I have also noticed that a lot of older women will have more difficulty growing their hair longer once they have chopped it all off. I would personally keep my long hair because it will take forever to grown back and also people tend to lose their hair when they get to a certain age (everyone's different).