The two most likely theories I've seen are:
1) Blue Mage is a solo class that doesn't fit into the rigid role system of FFXIV. By learning various blue magic, BLU will be able to do all kinds of things that any other single job wouldn't be able to, but since it doesn't fit into the classic DPS, Tank or Healer categories, it won't be able to do group instances. We've already seen it in both Cutter's Cry and Akh Afah Amphitheater, so perhaps those are "BLU-only" versions of those duties. It's possible the new Trust system could also be used to augment the solo play.
2) Blue Mage is a group class, and the level cap will be raised to 80 when Shadowbringers is released. Because going around the world learning blue magic is a time consuming process, the devs want to give us this opportunity before the expansion to take our time and build up our BLUs. If they released BLU with 5.0, players might be overwhelmed trying to gather blue magic spells, on top of doing all of the other things in the new expansion. It could also be that because BLU is a more complicated job than anything they've released before, they're taking the time between 4.5 and 5.0 to "beta test" the job so that they can fine tune it before Shadowbringer's release.
Hopefully we'll find out more tomorrow.
