I will like to say it is by copyright issues, they might get lucky and let it be on the album, like I don’t think Taylor Swift will be on NOW 118 but I put it on my predictions just for out of the blue, they might let her on NOW 118 but I doubt it, plus R2 songs, not everybody likes R2, I only listen to it because of the quiz at 10:30am, maybe they done that to get new listeners a taste of the new albums that they artists released and then listen to them as well.
For the US NOWs, from NOW 1 to 31 was just hits from US, and then from NOW 32, they started to release the album with a NOW What’s Next! track, which is totally new and might get released on the next release. Since NOW 33 to NOW 86, they done 16 top hits and then 4-6 NOW What’s Next! tracks to make them popular and then NOW 87 to today, it is like a few NOW What’s Next! but have more songs for the top hits. I just think of it this way with the R2 songs