Dual layer UMDs apparently have an upper size limit of 1.8Gb (which is gigabits, not gigabytes). Many games were limited in their potential size because they would otherwise become larger than would fit on a UMD, so there is this odd chance that in order for some games to reach maximum potential they would require only a limitless sized digital format.
CHEE5E, you make me rest assured that there may be releases in both formats. I'm in high expectations for the future that the PSN store should support all PSP models.