It was Bishop vs Anderson, not Bishop vs Lehner.
At the time, Anderson was still in his prime considering goalies tend to play at later ages. He was the first stable goalie we had since Lalime. Bishop was still relatively unproven as a full time starter.
Bishop would have had to be willing to sign an extension prior to the TDL because if he did not get enough starts here he would have become a UFA. We don't know if he would have been willing to extend here with Anderson in the picture. He probably wanted to go somewhere that he'd get an opportunity to start.
We almost got Couturier from PHI for Bishop. Instead, we got what was supposed to be a good young forward in Conacher who helped add a bit of scoring down the stretch to get us into the playoffs. Unfortunately, that good young forward wasn't as good as we thought he'd be long term.
With the benefit of hindsight, keeping Bishop and dealing Anderson would have been better. But at the time, keeping Anderson and using Bishop to get help up front made sense considering that I doubt keeping both was a viable solution.