Airport , TimeCapsule , displays, and AppleTV are in similar state. The Mac mini to some extent also.
Most of the stuff leaked is by second/third level parts manufacturers/suppliers who have lots of employees roaming around their factories. The machines with lower second/third level volume don't. They'll loose the business if they blab, so they keep the parts secured.
There is probably a relatively very small team at Apple working on the Mac Pro. Some of them may not even work on it full time. If only a very small group knows the details, it is hard for a tattletale to leak the info.
I suspect the "may not make a next version" leak several months ago was one of those backdoor leaks that Apple does from time to time though.
