Because healers and tanks don’t have access to all the other positive feedback that DPS has from the in-game system. DPS have access to way more medals (which give you access to XP) than healers and tanks, for example. Tanks can maybe get objective time (maybe) and healers can get healing, and that’s about it. DPS can get kills, objective kills and possibly take objective time from the tank depending on which tank/dps combo we’re talking.
Not to mention how much more fire they are awarded for doing their job well, how much more often they get POTG, and the various audio cues inside the game that make doing their job well much more exciting. You never hear the announcer screaming “TEAM SAVE,” like they do on a Team Kill, even if Transcendence just stopped a wipe, for example.
Don’t get me wrong, if there was an absolutely bonkers DPS on our team, I may vote them.
But my votes are mostly going to healers or tanks as as result of the DPS bias for the rest of the game’s kudos.