Why was the bridge placed to the right in the first place? Could they not have placed it in its final position when construction started? (My apologies if this is a stupid question: non civil engineer here)
The bridge on the left used to be a two way bridge.
They could not construct the bridge right next to it since they would have had to close the original bridge (since it was so close to the new construction).
So the built it off to the side. Once it was built they opened it to traffic (temporarily) presumably to get a head start on converting the nearby roads to match the new traffic flow (which they would need to do anyway).
Then they slid it into place and the new traffic pattern is already set up. They actually could have left it off to the side but that wouldn't match the existing roads as well (too many turns).