I don't think that has anything to do with it: I count 52 tracks for the OST, and 46 for this album. The only track that is remixed more than once on EOB is Rydia. The reason the remix album is longer than the OST is because overall each song is longer, due to the nature of remixing, which involves extending sections, writing new material, changing keys, tempos, styles, etc. etc.
Unless you mean that there weren't enough GOOD songs on the original soundtrack to do a full remix album like this, which is entirely subjective.