i think it had something to do with royalties. That's why you'll never see a Beatles track by the Beatles (as opposed to a cover version) on a compilation CD etc.. It's too expensive!
The recently late Neil Aspinall, who oversaw all copyright asspects of the Beatles' catalogue ensured that permission was never given for the use of any of their music in adverts, or in compilations, or in fact for any use except their own.
It has ensured that their legacy remains untainted by advertising dog food, or Big Brother, or what ever.
Although certainly not commonplace, Beatles tracks such as "All You Need Is Love", "Nowhere Man", "Across The Universe" and "Ain't She Sweet" amongst others have found their way onto compilation albums, including releases by 70s stalwarts K-Tel and Ronco.
Glad I'm not the only one who thought that was strange when I was searching on Itunes for free as a bird the other day. Took me long enough to remember the title and it was released in the 90s!