Well they did show a Wireframe of it.
But there really was no reason to show it on screen, since all they needed to know was how it crossed over.
Plus they probably want the first true appearance of a Constitution class to be in the Prime Universe.
DSC isn't a sci-fi show that deals in showing the spaceships it talks about on screen. How often did we get a scene now where a ships closes in on the Discovery but we do net get one single shot of it?