Why didn't these songs become standards?
I agree with Pillowrock...
"Seven Brides for Seven Brothers does not have one of those songs that crossed over into the wider, more mainstream culture beyond fans of musicals."
But it does have some of the most memorable original songs for a film ever. Most of them are very sing-able and you like them the first time you hear them. The lyrics by Johnny Mercer are without equal, especially in "Spring Spring Spring". "Lonesome Polecat" is a treasure. My favorite is "Wonderful Wonderful Day". But I hate "When we're in Love", find it too sappy. It is surprising none became standards, so many are first rate.
The only other original songs for a film of the 1950s as good as these that I can think of are in "Hans Christian Anderson", 1952 with Danny Kaye, and several of them became standards. "Anywhere I Wander", No Two People Have Ever Been So in Love", "Wonderful Copenhagen" etc.