Autistic people can have a lot of difficulties and can be severely impaired in daily life. They can feel highly anxious everyday. The sad thing is that even the people closest to them may not know what they are going through.
I suppose there is a similarity here to some of the experiences that depressed people have. In the media, we often hear stories of people who have struggled severely with their mental health but that their family and closest friends were unaware.
This brings me to the first reason why an autistic person might be unable to tell their family how they are feeling: