If y'all remember, the driveable alpha prototype of Model S, when the first "spy pics" of it caught in the wild on the road were taken, *had no interior*. Loose wiring everywhere.
If that's the stage they're at with Model 3, they might be willing to show photos of the exterior of an actual prototype, but totally unwilling to let anyone get up on stage and look inside.
Even if the interior is in the state which the Model S interior was in at the formal reveal (with completely different seats from the production seats, among other things), they might not want people looking too much at the inside.
If I were to guess based on Tesla's history, I'd guess the exterior is pretty much designed and the prototype is driveable, but they may have a lot of interior decisions left to make. To avoid confusion they might not show off a totally-not-final interior.