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Many portals go with G because that's the brand most people want their search coming from. If they can't get it from their portal's home page they'll go straight to Google. And a portal doesn't want that.
AV has been mentioned quite a bit as having beefed up it's results, and is presenting some quality serps. Looks a lot more active and inclusive than AJ at the moment.
LS is in a fight for survival and thus hungrier, it doesn't compete with it's partners, and would make a good match with a portal if it ever gets around to dusting off wisenut and getting it into shape.
AJ announced a hiring spree last month, mostly for sales/marketing team so you never know...