I’d say there’s definitely something in the suggestion that it could be a contractual clause that meant any compilation had to include a self-penned track.
The first Greatest Hits had Changes, the second (in some countries) had The Promise, and the UK/EMI 1982 hybrid had both Changes and Rosewater. None of the three songs had been A-sides. All fair enough really, and I love the fact that she had a lot of control.
Summer Nights was already a part of Grease, whereas You’re The One That I Want and Hopelessly Devoted To You were much more hers as they were written for the film by John Farrar. Whether that had any bearing is anyone’s guess. I’d agree that as other cast members appeared on Summer Nights, it might have made things more complicated, though it was included on that UK version (with 20 tracks, its exclusion would have seemed even more odd!).
One thing that’s interesting about the singles from Grease is that You’re The One That I Want came first. We saw Sandy 2 before Sandy 1. And by extension, two different versions of Olivia. No one batted an eyelid, which is credit to Olivia’s ability to transform herself so convincingly.
I do remember the Guinness Book of British Hit Singles reminding us that John and Olivia as a duo were the most successful chart act in that both their first and second singles were number one, their first and last singles were number one, and they reached the top with every single they ever released. These chart statistics were spoilt by the release of the Grease Megamix in 1990.