I hear what you're saying, but based on how much cover there is in this game, I'm almost willing to bet a weeks pay that they tried prone, and found that it totally screwed up the dynamic of the game.
As someone pointed out earlier, it's not always best to mimic reality to make a game that's fun and balanced. With all that cover out there it would make sniping almost godlike. In previous BF games, there were no "destroyable" environments, and almost no foliage and ground cover, so Prone players still stuck out to players with as little as an ACOG. not so much the case here.
While it's just my opinion, I think that it would have slowed the game play to a crawl, and people would be up-in-arms over a completely different issue--not being able to move without getting shot.^D^
"Never start a fight, but always finish it."