I'll throw my $.02 out there...
Another factor could be that Tesla might have a systematic method for determining SC placements. They must have some sort of "master plan;" just looking at the current, 2015, and 2016 maps indicates as such. I am sure that the process is extremely easy in many locations, but excruciatingly difficult and frustrating in others. (Bishop, California comes to mind.) Tesla assumes that there will be an "average" of X days from initial discussion and planning to completion. They pursue those routes and locations according to their plan. When issues arise, they work to correct them to try to get these highways and areas open first. If an area (like Bishop) becomes unworkable (cannot find a suitable location with reasonable rent) during the early parts of their planning, it gets tabled until the next round. This may be partly due to the final determination of Supercharger placements. The Supercharger team may have to rework their locations and spacing if one planned city falls off the map, as it were.
I do not think that Tesla is saying, "Well, since we are experiencing problems along this route, we will shift our emphasis to this other route instead," just so they can meet a "number" of completed Superchargers. And, it does not make any economic sense to build isolated, outlying Superchargers today, if access to them (say, Bismarck, North Dakota) won't be for another 12-18 months.