One factor could be that the prominent links into G+ across Google properties means that people are less prone to navigate into G+ via by with Google search as

One factor could be that the prominent links into G+ across Google properties means that people are less prone to navigate into G+ via by with Google search as they would do for Facebook/Twiiter. Most peoples initial contact with it would've been the "+You" promotion with giant arrow pointing towards the black navbar, skewing the results even further against searching for "Google+" inside of Google.