The Knight job was the most unique job among all the other jobs. This job could fight and could deal damage. It was basically a standard jack of all trades.
Moreover, Knights had a unique halo skill which could increase the defense and attacks of teammates around him. Additionally, it could even cause damage to the target.
The more impressive aspect was that Knights could even make use of the power of talismans, change the shapes of weapons, use any type of swords and spears and could even turn weapons into gloves set to fight like a Pugilist.
Because of all these possibilities, it became incredibly complicated to fight as a Knight. Players with insufficient capabilities would be flustered while playing this job. Therefore, most players would rather choose to become a Warrior, which was a much simpler version of the Knight.
This was the reason why Knights were as uncommon as Pugilist among players.