Because they only have 25 bids to NorCal, and in the past they've had 12 or more D6 entries, and so gave D6 four NorCal berths.
Backing up ...
To get the Open passed after it was voted down the first year, it was necessary to make sure there were enough votes in year two. D6 is the largest division in number of votes, so obviously it was critical to get their support. Thus the four bids.
This year, there were only eight D6 girls teams interested in participating, and one forfeited. (Travel may be the issue -- I don't know.) There were 12 boys teams, though, so D6 schools will never vote to going down to two berths.
It doesn't seem likely that they would have different criteria for boys and girls, but it does seem foolish that Ferndale advanced to NorCals on the girls side without playing a game.
Why NorCals won't go to play-in rounds and hand out more bids is a mystery to me ...