At the June 13th Special Long Beach City Council Study Session (video link posted below) Long Beach Advance Planning Officer Christopher Koontz answered that question. It’s at the 22:00 mark on the video.

COUNCIL MEMBER SUZIE PRICE: Why is it that we look to established cities in talking about increasing density as opposed to establishing cities in areas where there is currently no development or there is ample amount of land?
ADVANCE PLANNING OFFICER CHRISTOPHER KOONTZ: Sure that’s an excellent question council member. So state planning law has really evolved considerably over the last ten years and it’s very much driven by greenhouse gas emissions and when you develop a new city on vacant land or you expand on vacant land, and we do see that activity still happening in the far reaches of the inland empire and Kern County to the north of us here but the reason that’s not as desirable under California law is that new development by definition because it’s outside of established centers the amount of driving that those future residents are going to do to employment centers but also to the basic services is going to be considerably more than if they’re located within existing cities with access to existing infrastructure and existing goods and services and we have much work left to do but our emissions are per capita emissions in a place like Long Beach are orders of magnitude less than in places that are now cities but were not too long ago built as new development places like Valencia out in the northern part of L.A. County. So that’s the driving force I think that the legislators have in mind when they made these changes and then from the staff table we don’t get to make those decisions. We’re just trying to implement those state mandates here at the local level.
http://longbeach.granicus.com/MediaPlayer.php?view_id=12&clip_id=9191