Its because most people think they are playing a deathmatch, they want kills cos the big numbers at the end of the game makes them feel special. They are the first to complain though when their team isn't doing good, because everybody looks bad when your team isn't making it happen. Too many players are happier with a lost game where they had 25+ kills than a win with 5-10 kills, its a very egocentric crowd we have here.
The only way i see this changing is if you combined kills and assists as only one stat, or at the very least up the rewards for assists a little to encourage tanks.