I think the problem they're predicting, which another poster pointed out, which could be why they aren't proceeding with mass production of this particular item is that they aren't sure how much life such a product would have in the long run.

Should they mass produce a thousand, ten thousand, twenty thousand..and only sell a quarter, maybe half of that, so that the remainder sits in storage because no one else wants it?

That's unnecessary cost on their hands, plus storage space issues, plus labor to create the shirts. Even if they pass the print off to a third party maker and have them produce it, they're going to have to pay that third party to make the shirts for them, plus it goes back into a storage question and how much is enough, how much is too much.

Whereas if they do create a set limit of shirts and then break up those quantities based on events they're planning to give them out too, they can have a set inventory and not worry about going overboard.

The only real problem in that method is that other companies (I've already seen on) is taking it upon themselves to remake the shirt themselves and from what I can tell giving nothing back to SE on it.

What they -could- do is create a limited time offer, have a specific number of shirts produced and offer it through their store to customers who will buy directly from them, first come first serve and once quantities run out, that's it, no more are made. Of course, such a method will tick someone off in the end because there will be at least one person who missed the boat. (probably me lol)