Walla Walla River Bridge
Photo taken by Mike Goff in July 2016
BH Photo #357976
Maybe they wanted a bridge with higher elevation in case the lake filled up during high water events. Its also worth noting that the new bridge was a realignment for the highway too so it may have afforded a better curve for the highway. It was not unheard of for bridges in this period of rapid development of highways to have a short service life as highways were realigned to allow for safer and higher speeds.
It seems it had something to do with filling in the nearby lake, according to this guy:
Anyone have the story on why this bridge was in use for so little time?