Some I will say lack of been able to trust their individuals again due to the past experience they might have encountered.
Some may have been duped, cheated and this may have made them to conclude in their heart that they can not trust anybody again so instead they preferred to be in isolation than been duped or cheated again.
Some are actually born with it or let me say since their little age. They lack the habit of interaction or engagement with others either both close or far to them and this continue to when they grew up and them they get used to it.
So this may also contributes.
This are only the two reasons I may say contributes. The fear if been duped or cheated again and the lack of having the habit of interaction.