I already knew all of thisI can't speak for Chuck, but IMHO, the thing to remember is "the store" isn't intended like a real store where you walk in and everything is available in every size, color, or pattern. The store is there to offer a limited number of HT related items as they become available.
Chuck surely did the best he could, within a reasonable price range. It's my understanding that the t-shirts were simply not available in larger sizes from his supplier. Plus the design of the t-shirts were reasonable and recognizable by others. I would also assume that Chuck did not want to financially commit to more t-shirts than he knew he could easily sell.
There has been discussion about other items for sale, but there's a great deal of coordinating to do (it's a big job); plus the commitment of funds is also a consideration.