It's because CraftBukkit (the server mod that you guys play on) is simply incapable of releasing the new version until sometime after the Minecraft client update goes public. In all honestly, you guys are looking at a week to two weeks before CraftBukkit gets the right version for you to play 1.8 on it. A beta version of CraftBukkit may be released earlier, but it will most certainly contain bugs.

Declaring the restart to happen on the day the update arrives was, quite literally, the dumbest declaration I've seen in a long while.