Why a person when joining an organization preferring his previous colleagues for any position in his new organization even having best other options?

Its because people love familiarity. It is normal for every person to want to work with people they think they know in order to be comfortable.

 

However, its unethnical because sometimes they do not deserve those positions.

Because he is not know the attitude of thier new collegues therefore he is trying to position in new orgnization