Based off of my own experiences and my family's, here's what I know:
Some people may be hesitant to ask for help from friends and family because they are scared to be judged by people they know. They probably would feel more comfortable talking to someone they don't really know because they have nothing to lose. They don't know the person, so they probably wouldn't worry so much about being judged or disliked by this person they don't know personally. This is just the general idea, but all people are different, so why a person may open up to family and friends less than someone they know varies between the person and their feelings. For example, someone may not want to talk to family because their family doesn't care, which is quite sad, but it happens.