Well, traditionally Clerics in D&D used to be able to wear heavy armor, every last of them.
Now they only get medium armor and they have limited it to certain domains that still get access to heavy armor.
Why ? I simply dont know.
But War and Tempest are both very aggressive choices so it makes sense to me they would be heavy armor and martial weapons.
Life obviously is a very defensive choice, so heavy armor makes sense, in some way.
Nature, I dont know. Balance issues ?
If I was asked to design D&D6, I would probably give Clerics and Druids a relatively small set of core functionality and add to that depending upon what faith the Cleric or Druid follows.
A Cleric of Shar for example would get light armor and spear proficiency.
