The reason days was bundled with I.5 was because a KH1FM, ReCoM, KH2FM I.5 bundle and a Days, ReCoded, BBSFM II.5 bundle would result in drastically lowered sales for the second. Plus it's unbalanced in weight. Critical story games are KH1, KH2, BBS, 3D, and CoM (mainly for Reverse/Rebirth). Lore expansion games are Coded and Days. Of unknown importance are [chi]/Unchained/Back Cover and 0.2 a fragmentary passage. So logically it wouldn't make sense to have one super important collection and one mostly unimportant collection.
TL;DR Release order.