the only metric you have is ranking tier for matchmaking. if you have to add more things like say damage done, kills, etc. it's more information that has to be stored on your character and would most likely lead to increased queue time which is what they probably don't want to happen.

this is only the first season, so yes hopefully they can refine things a bit more but it's still very much a work in progress so you can't really say things like like delete PVP.