I reckon it was a financial decision. If my own memory serves, the plan for the Purple Rain tour was for Apollonia 6, Sheila E., and The Time to serve as the opening acts for Prince and the Revolution (as Vanity 6 had done on the 1999 tour, and as the Time had done on the Controversy and 1999 tours). And in order to tour, you (arguably) needed to have an album out to tour behind, hence the Apollonia 6 tracks ended up on their eponymous album, and the Time tunes were released on their pretty excellent Ice Cream Castles LP.
Of course, then Morris decided to leave the Time, so that cut the touring package down to three acts instead of four (or two acts when I saw the tour: Sheila E. and Prince and the Revolution).
EDIT: And I’ve no idea why Dez Dickerson and the Modernaires never got a release of any kind (save a few seconds in the film).
I wasn’t a huge fan until I met my husband, who had all of Prince’s albums at the time. I got to know his catalogue, and I was a complete convert. My favourite songs and albums of his aren’t well known at all (I did a quick look to see which album I would consider my favourite, and it’s about a five-way tie).
My point is, it’s really worth it to get to know the rest of his music.