Because... it's not much of a prestigious belt if any old wrestler get's a shot every week on TV. They're trying to build the belt to mean something, and having it defended on TV every week would not help towards that aim at all as surely not everyone who would be having these title shots would be anywhere near deserving.
Plus, the idea then is that the NWA title match becomes something special and merely the fact the belt is on the line and the Champ is in action can help draw a bigger buyrate, or, if needed, pop a TV rating for a special Impact! or something.
It's the same reason Flair didn't defend the NWA title on WorldWide or whatever every week during the 1980s.
However, Christian does need to be on Impact more, even if it's just doing commentary or something on a match to keep him - and the title - in the minds of the fans.