Hardly seen in U.S. theaters?
At Buffy Sainte-Marie's website, she discusses Soldier Blue a little (link below). She says that while it was pretty popular in Canada and Europe, it was fairly obscure in the U.S.
http://www.creative-native.com/lyrics/soldier.htm
As I was watching the film for the first time recently, I was surprised that this movie was apparently widely released in the U.S. back then--for that matter, it's hard to imagine it getting a wide release even today--but apparently the release wasn't so wide.
I'm guessing that there were probably some parts of the country where it didn't play at all.
I wonder if there were political reasons for Buffy Ste.-Marie not getting an Oscar for her song (and apparently not even being nominated)? The gov't had (and continues to have) a fair amount of pull over TV/movies. She described in one interview (viewable on You Tube) that President Johnson's administration leaned on TV producers to prohibit her from playing certain songs.