The concept of a "Premium" did not exist at the time Tron was released.

Before Tron, there was only Limited Edition.

Tron Legacy was Stern's first attempt at releasing a Limited Edition version and a Pro Version at roughly the same time.

I do not have any inside information or solid "facts" to back up the following. It is just a possible reason.

Once Tron LE became a hit, Stern had painted itself into a corner. The Tron LE was marketed as 1 of 400. The significant hardware differences included a moving recognizer, drop targets and the colored fiber optic tubes framing the ramps. The software differences included DAFT PUNK Multiball, a way to earn an extra ball and an integrated light show using the fiber optic tubes. So Stern had to choose between possibly upsetting the 400 Tron LE owner by adding a new Tron Premium line that would bring joy to X new owners. Without knowing the value of X, Stern decided to play it safe.

Stern could have released a premium version that used a different backglass or cabinet armor and earned some additional sales. It also had the possibility of alienating the 400 buyers who bought Stern LE believing it would 1 of 400.

Rather than risk losing the collector market on future machines, Stern made the decision to learn from their mistake, sacrifice additional Tron Premium sales and focus on selling the Tron Pro.

Granted, its quite possible that Stern underestimated the popularity of the Tron Theme and simply failed to order enough parts to create a Tron Premium.

Marcus