So that casters don't become even more powerful. When things hit the hour/level mark, they become all day buffs, which is quite convenient because you'll be able to cast them again tomorrow. So when you start having all-day spells they effectively become permanent, except without needing to spend a level 5 spell and several thousand gp casting permanency.
Yes I follow that but why not Mage Armor and Magic Vestment which are still 1 hour/level? Or have they also been toned down to 1min/level in some errata?
