The Packers running game, because of their zone blocking, basically aounts to hitting the line and making one cut. The had to adjust that with Benson because he is more of a smash-mouth kind of guy, but the hit it and cut style is what both Lacy and Franklin do best, and Harris showed he could do that as well. I think that fits in better with what they want to do on the ground.
In addition, Lacy in particular is supposed to be a good blitz blocker, which neither Starks nor Green developed into.
I think with the limitations Stark and Green have and in particular the injury history on Starks, I don't see either guy making the final cut unless there are injuries or unless they really tear it up in the preseason.
"The difference between genius and stupidity is that genius has its limits" --Albert Einstein