I don't think it's an issue of the source material - I suspect it's more to do with getting permission from Carrie Fisher and Mark Hamill and Harrison Ford, etc.
"Hi-def" is a bit of a misnomer - what it really means is that the films are HIGHER-def than if you watch them on the Atomfilms site. Maybe a few of the films are actually Hi-def, but Pink Five certainly wasn't one of them, it's plain old NTSC.