That is excessively easy to explain... with very few exceptions Armor is increased *only* by going with a higher iLevel item. As such it isn't really something you deliberately seek out - most of the items you get in a given level of content are the same iLevel and will therefore have the same Armor.
Armor is almost always only something to think about as a part of "Do I go with this new iLevel X item or keep my old iLevel X-5?" type comparisons. Even then it is likely to be overshadowed by differences in how the stats are allocated on the item.

