Welcome to the Spotify community and thanks for reaching out about this here.
I recommend you read this article.
Note: the music request form linked at the article is not active anymore.
Ordinarily, unavailable music is almost always a decision by the artist/label. Availability can vary over time and between countries, depending on the permissions from rights holders.
Maybe you can ask the artist why the soundtrack is not on Spotify and suggest it to him/her.
Spotify adds new content every day, so if you can’t find something you’re looking for right now, it may appear (or reappear) soon.