F2P does not mix well with low pop lobby matches. If it were the original open world game we were told long ago, F2P players would have mixed in fine in the battle field.
You cant have only orks be f2p becaus A: that screws them, then they quit en masse, and it has solved nothing. And B: if only one race could be f2p the only one that could actually attract players is the marines. And we can see how them being the most popular with new players is working out for them.