It hasn't been in there from the start, because it makes little sense with just two factions. If you are searching Ostheer and the Ostheer search percentage is over 50%, you can simply switch factions, and vice versa.
Huh?
I just want an interesting game, whatever the side. And the percentage of people queuing isn't everything either, nor do I want to be forced to change every five seconds...
Random factions makes perfect sense as long as there is more than one faction.