Because as the number 10 is the creator and player that most attacking play is going to go through, you need to see some kind of ability and awareness to warrant a player to be given that role. Its the position most players want in modern game, so to give the player that role above his peers then there needs to be a reason. Therefore you have to show your peers you warrant that position.
In terms of us
Dembele has shown that if you play the ball into him he is strong and doesnt lose the ball - Possibly promotion
Kane has shown vision when he has dropped back into that position creating erikksen's chance v utd and Davies ball for sats goal
Erikksen started on the left and has been our most creative player
Chadli has offered a constant goal threat and has proved to be a calm finisher
Pritchard, Carroll have been playing in a wider role and havent been around the club long enough to push those 4
Lamela has played 38 games and scored 2 goals, less goals than the allegedly "wasteful" lennon in every season he has played for us bar 1 in similar number of games.