Shyness, insecurity, or lack of knowledge of how to communicate.
Sometimes not sure
To often they are afraid of being vulnerable or hurting other people
So people without question but that's only the really sucky ones
Inside we are all unsure of ourselves. Well some people know what they want to say is nasty or wrong so they don't say it. Others think why bother they won't listen therefore giving neither person a chance to understand.
I always said "communication is impossible" "Communication is improbable" "communications is so unlikely"