According to Marc Ambinder
http://marcambinder.theatlantic.com/...
I think it was necessary at this point because it was becoming too much of a distraction.
Here is Johnson's statement courtesy of First Read:
http://firstread.msnbc.msn.com/...
"Jim did not want to distract in any way from the very important task of gathering information about my vice presidential nominee, so he has made a decision to step aside that I accept," Obama said in a statement. "We have a very good selection process underway, and I am confident that it will produce a number of highly qualified candidates for me to choose from in the weeks ahead. I remain grateful to Jim for his service and his efforts in this process."