The FM movie was around 1978, but there is an earlier compilation called "BBC TV's Old Grey whistle test" from 1975. It features "Keep yourself alive" as track 2.BTW. The first compilation I ever saw a genuine Queen track on, was the soundtrack to the movie FM, which had We Will Rock You somewhere in the middle of it. Fairly sure it was the very first, in the UK anyway.
Some Queen book in those days had the information, that this record featured a different version of this song, that's why I kept searching for it many many many years ago. But that turned out not to be true, when I finally found it.
The policy of "being the first" on a compilation may have the simple reason, that they wanted to have the best sound. Vinyl albums have the best quality at the beginning, cause with each round the length of the groove is getting shorter. And very often early samplers had more than 25 Minutes on one side, which in general lead to a poorer sound. So being at the beginning had the lowest risk of sounding muddy and distorted. I have later CDs, where Queen is on 4th position or even later. So they may have given up that policy at some point in time.