Community Manager Bornakk responds that Zul'Aman gear is not meant to be a full replacement of Karazhan tanking gear. In fact, the low occurrence of Defense was intentional. Instead Blizzard wanted to offer pieces that filled other stat gaps to compliment alternate tanking styles. Examples are Hit Rating, Stamina and Block Value.
While it would be nice to have a full set of armor catering to our exact needs, the reality is that there are different styles of tanking (as well as healing, dps, etc.) Blizzard has chosen to service all those styles with their current system of modular mixing and matching. And if equipped armor doesn't put your stats where you want them to be, there is also the gem socket system to customize your toon even further.
Moral of the story? Don't expect a custom suit off the rack. An outfit that is going to fit you like a second skin is going to come from attention to detail on your part.