Psychology could cause mediocrity and bad experience could lead to not make any action to new opportunities. 

Maybe there are more than three but let's write three of them:

1- Being Afraid: People are afraid to seem like unsuccessful and they do not make any new action. This is due to possibility of encountering bad results. They choose to be an ordinary person and not try anything.

2-Feeling Tired: They could have many experiences and that is why that could lead them be a lazy person due to the failing before

3-Being Inpatient: Some people do not have enough energy to complete anything they have started. That causes being bored and killing excitement they have. 

These could be reason to mediocrity and could lead people behave like that.